Galactus was supposed to be a one-time only visitor, but fan reaction made Stan bring him back.
Saying no to fans on occasion would have changed the entire comic business.
Squirrel Girl’s “fight” with Galactus was one of my favorite things I’ve read in years. She’s a comic character with optimism, a sense of humor, a kind heart, and common sense. It’s kind of amazing how unique that is.
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Yeah, Squirrel Girl is intended for comedy, much like Nextwave or the Impossible Man. (Is the Impossible Man even still around?) Nothing she does should be taken as canonical… As if that that word has any meaning for comics to begin with.
She was the last Marvel character created by STEVE DITKO so she’s a legit character and the Watcher himself said her beating up Dr. Doom and Thanos were both canonical (and not robots/clones/space phantoms). Canonical. So there. 
Of course, we fans are perfectly capable of ignoring canonical content we don’t like.
(In the current Dr. Strange comic, it is “established” that Doc can’t eat normal food, but has to force himself to cram down horrible magical garbage. You know something? Bullshit. I don’t buy it, I don’t accept it, and I hope some future writer throws that bit away, depicting Doc having a nice dinner at a restaurant somewhere.)
(I also wish he were still love-bunnies with Clea. Sigh…)