William Shatner was just fine in the role of Kirk.
I think there’s some confusion here between the notion of sidekicks being zanier and funnier than the main characters - which is true of most shows - and sidekicks unintentionally being way better characters than the mains, which isn’t supposed to happen.
Many shows are SUPPOSED to have “funnier” supporting characters. Seinfeld’s supposed to be the way it is. Jerry is the straight man, and everyone else represents his observations of how weird manners are, just like his standup routine. “Everyone Loves Raymond” is the same way; Raymond is supposed to just be stupid. Everyone else is funny because they’re the crazy relatives the stupid guy can’t understand. This However, some shows place the font of wackiness in the main character; in “Home Improvement,” Tim is supposed to be the funniest guy on the show. “Frasier” stars a character who is supposed to be a pompous fool who’s always humiliated.
“Will and Grace” is, IMHO, a case of a show where the supporting characters are funnier than the main characters but aren’t supposed to be. The show is such that one of the characters (Grace) is so weak and boring that the show’s intended central source of laughs (the Will-Grace relationship) isn’t as funny as the zany sidekicks.
Look at it this way; if you got rid of Tim Allen, “Home Improvement” wouldn’t work. If you got rid of Kelsey Grammer, “Frasier” wouldn’t work. If you got rid of Matt LeBlanc, you’d be surprised how much that would hurt “Friends.” Topher Grace is a pure straight man on “That 70’s Show” but if you dumped him the show would be too zany, and wouldn’t work. But if you dumped Debra Messing, I don’t think “Will and Grace” would skip a beat. Hell, “Will” might be a better show. And that’s why Megan Mullaly and Sean Hayes get so much attention; without them it’d be mediocre.