I have no problem at all with people liking something like that, but it's just not for me. I have wondered occasionally how it got so popular, it seems to be everywhere.
I remember it mostly from the 80s, as a toy marketed for girls like under age 8. I can understand why adults might like kids entertainment out of nostalgia both for a certain toy/show/book etc and for childhood itself.
For me personally, this whole "cute" thing is a bit weird. I find it kind of offputting, which is why I don't like Disney stuff and most Manga. I kinda have the Animation Ghetto thing in my head, I enjoy some of it, but think of it as mostly kids stuff. I enjoy playing with a real kitten or puppy, but I rarely watch videos of kittens and stuff like that.
I have a provisional working theory why people like that kind of stuff. All youth/subcultures I have been involved in have taken great lengths to be as offensive and disturbing as possible, politics, religion, violence, sex, you name it. I have often discussed with friends that nothing could come after us that could shock us anymore and nothing could go more extreme. But now young people like this "cute" stuff. It's so cute and happy that I find it almost disturbing. So I guess this could be in part some kind of counter-movement. I mean an otherwise "cool" looking 16 year old playing with a MLP toy (something a teacher friend of mine observed) is as weird to me as 16 year old me was weird to my parents with all the loud music and satanism and violence and stuff.
But maybe I'm just old and cynical.
I do think there are parallels in how people perceive MLP fans and fans of other, older or completely different subcultures, in that they often seem to assume the worst without actually trying to get a clearer picture.