The Null-Frequency Impulser seems to be by James Nelson Coleman. One site lists him among “African-American Authors”, something I was unaware of. But it’s still not a good book.
Cal, I suppose “mediocre” or “run-of-the-mill” or “average” would have been better words for my thread title. The ones you list are not enjoyable or fun in any way, except perhaps after a few drinks and mind-altering drugs, and perhaps just to have fun trying to read aloud without giggling or groaning.
I’m surprised by some of the choices as well, but it is an opinion thread, after all.
I was expecting more names like Nolan and Johnson and Bunch and Anthony (MacIntosh, Phillifent, K. M. O’Donnell (Malzeberg, slumming, iirc), etc.
Sir Rhosis
By Klotho’s bloody blaster - Doc. Smith is my favorite. I read the entire Lensman series not too long ago, and they’re still fun.
I did reread Galaxy 666, but I found I remembered it very wel from my first time reading it 30 years before. It sticks with you - kind of like skunk. 
Ron Goulart wrote a funny book about silly robots. Then he wrote it about 20 more times, changing names and locales but not much else. The first book wouldn’t qualify him as a bad author. Books 3-20 (or however many there were) DO qualify him as such.
Cal, I chose my names as authors whom I like to read. I can’t say that I ever really enjoyed reading “Eye of Argon” except as a challenge to keep a straight face. I mean that is soooo bad… Umm… Anyways…
I read the OP as a request for authors that one reads even while feeling a little embarassed for doing it. Not authors that one reads at home. With the doors locked. With the curtains drawn. With the book in a brown paper wrapper (or perhaps a John Norman Gor cover pasted on.) so no one knows what you’re willingly exposing your precious bodily fluids to. 
I’m not about to suggest that any of my authors have produced, even on their worst day, a stinker that deserves to be mentioned even in the same paragraph as “Eye of Argon” or Perry Rhodan. 
The Fionavar Tapestry.
As my username attests, I’m a huge fan of the work of Guy Gavriel Kay. Tigana, Arbonne and Al-Rassan are among my all-time favorite books, and while I have some issues with his more recent work, it’s still head and shoulders above the vast majority of modern Fantasy. His first trilogy, though…
You see, Kay was a young Canadian law student when he was approached by Christopher Tolkien, a friend of his family, to help him edit his father’s vast collection of notes into what would eventually become the Silmarillion. It is my theory that after several years poring over the Professor’s endless excursions into Northern European myth and folklore, his head exploded. There happened to be some blank pages about, and the result was the Tapestry.
The book has it all. Elves! Dwarves! Orcs and trolls! Two different types of dragons! Vengeful forest spirits! A werewolf! Giants! A Father-God with two ravens and a hanging tree! A Mother-Goddess with an all-female order of priestesses, who makes love to mortal men and then kills them to bring the spring! A Horned God! The Wild Hunt! Two completely unconnected adolescent boys who ride some form of flying horse! A horn that brings the end of the world! American Indians! Eastern Despots! Portentous poetry! Arthur, Guinevere and Lancelot (why not?)! And all, of course, seen through the eyes of five young Canadian law students.
It’s utterly ridiculous. As Fantasy literature it’s melodramatic, self-indulgent, contrived and - needless to say - highly derivative. And yet, despite the fact that it’s utterly indefensible, I still love it, because that sunofabitch can write. The plot may be supremely silly, but it sure does move. Many of the characters may be one-dimensional sketches, but they’re still compelling. The dialogue may be pretentious, but it just plain works.
I really want to hate these books. And every year or so I end up reading them again.
I’m going out on a limb and nominating The Lord Of The Rings - some of you may have heard of it. Yes, it’s tremendous fun. As a book {or three books, or six books}, it’s enormously flawed.
It’s appallingly structured: it takes forever to get started - you may be holding a ring which could decide the fate of the world. Hang on to it for a few chapters, while I go and investigate. OK, I’m back, and the Ring IS incredibly powerful and dangerous - you two amateurs leave the Shire immediately, carrying said perilous artifact - feel free to dawdle, and take some friends if you like - while I ride off again and leave you alone.
Conversely, it ends far too abruptly - oh, the Eagles have brought them back. Saved on some shoe leather, I suppose. Pity they weren’t around at the start of the trip, wasn’t it? Or did they only have non-smoking flights, and had to wait until the pipe-weed had run out?
There are far too many central characters: Merry and Pippin are practically interchangeable, and need to be forcibly separated in order to distinguish them. They also each have to be given an uber-villain to slay - at least one too many, in the case of Pippin. Still, I suppose he couldn’t leave the book with entirely un-bloodstained hands.
The villains are hopelessly incompetent, to the point of straining credulity: the Nazgul, all-feared undead wraiths, are fooled by the old “hide the pillows in the beds” trick. And the Witch-King Of Angmar seemingly hasn’t read Macbeth.
Plotting is hopelessly arbitrary, and relies heavily on characters making ludicrous strategic decisions simply in order to advance the story. Gandalf leaves the hobbits to cart the Ring, which controls the fate of Middle-earth, across the landscape by themselves? Aragorn abandons Sam and Frodo {still toting the Ring, and potentially the fate of the entire world} to their own devices, in order to quixotically pursue two kidnapped minor characters? Priorities, priorities!
And the quality of the writing? "‘Lose what you have found, lord?’ she answered; but she looked at him gravely and her eyes were kind. “I know not what in these days you have found that you could lose. But come, my friend, let us not speak of it! Let us not speak at all! I stand upon some dreadful brink, and it is utterly dark in the abyss before my feet, but whether there is any light behind me I cannot tell. For I cannot turn yet. I wait for some stroke of doom.’” Oh, kiss me, you mad, passionate brute!
Let the wailing and gnashing of teeth commence…
I had forgotten John Philifent until you mentioned him - I like King of Argent! Maybe it’s time to re-read it again…
And some of Jack Vance’s earlier stories are a bit ropey - Son of the Tree, Big Planet, Five Gold Rings and so on and also a lot of his short stories from the 40s and 50s - but I still like them and re-read most of his books over time.
I’m going to nominate R.A. Salvatore for his formulaic and predictable plots and super-human characters who always barely squeak by in a fight no matter how weak the enemy. Good ol’ Drizzt - only ever as strong as he needs to be.
A nod to the Hickman/Weis combo for giving us a plethora of 4th grade reading level DragonLance classics.
And, I’ll second David Weber. He can be a decent (if not imaginative) writer, but when he’s bad, he’s really, really bad.
Brian Daley’s Han Solo Star Wars novels from the late 70’s. I hang my head and weep while I say this.
Seriously, though, they’re good fun. He pretty much nailed Han Solo and the feel of swashbuckling space opera. Not something I’d read in public without first covering them with a brown paper cover, though–it basically is brain candy written to cash in on the Star Wars craze. But in a good way.