People don't like Teen Titans GO! because it isn't Teen Titans.
Like, the characters of The Amazing World of Gumball are one-dimensional and nobody complains about them because the cartoon is funny, and that's what it's meant to be -- if it's not your style of a humour then you shrug and ignore it. But people have a hard time looking at Teen Titans GO! that way -- they see it as a downgrade of the original characters rather than a different show.
I don't watch it regularly but I think Teen Titans GO! is often funny and there are episodes I like and laugh at and there are episodes I just sigh and am embarrassed. I'm not mad. But then, I lost interest in Teen Titans well before it went off the air.
I think that's probably the other thing. It's like seeing your favourite video game character get a Mii outfit in Super Smash Brothers and realising that this means you'll never see your fav character in Smash. You might complain about how stupid the Mii outfit looks, and maybe it does, but it's no worse than other Mii outfits -- what you're really mad about is that the Mii outfit's very existence means you can't get the character you wanted.
I can understand that.
I think these are all good reasons to say "I don't like the show," but maybe unfair reasons to say "the show is bad." I don't think it's poorly done; it's just doing something very different from what the Teen Titans audience wanted.