Undertale was more about the story and character interactions rather than character development, anyway. And Homestuck is… uh… honestly, I have no idea where the head and feet of whatever Homestuck's main appeal is, so, no opinion there, but it does look like simple characters work better than "oh so complex for you" ones there, too.

People in real life can also be overly simple – that does not make us any less likeable, regardless (not by itself, at least). The thing is, people like to overthink about fictional characters too much, probably to not be perceived as a "simpleton" who cannot like complex stuff or something, as if liking both was impossible. Like, if you enjoy complex characters like those on Fire Emblem, why should that stop you from enjoying simple ones like those on Undertale..? I do not see one being "superior" to the other. They are just 2 different approaches, not unlike how shows with a lot of action and epic stuff are not necessarily superior to laid back, "slice of life" stuff, and most people like both to at least a minor degree.

I agree with your viewpoint, but I think what the OP was trying to say (or at least how I interpreted it) was that they disliked the concept of Flanderization – where one character's originally quirky trait becomes what the whole character is centered around. Once it reaches that point, the character isn't actually a character anymore, just a walking joke. It's not even like they're a simple but fun character, instead they just have one thing going for them and that's it.

I won't argue for the other posters, because that's all up to opinion and interpretation and such, but OP does have a point.