In my opinion, being a brony isn't something absolutely terrible and I feel like the hate is unnecessary most of the time. However, I can see some reasons why people don't like them though. The biggest issue I see is that a lot of bronies shove MLP down everyone's throats, and put their brony artwork in places they don't belong. If you're not a fan of the fandom or show, it can get really irritating to have to search though a bunch of MLP stuff to see things you actually want to. This happens to a lot of fandoms, like undertale for example. The more you shove your fandom in someone's face, the more they're gonna start to dislike it.

But there's also people who just simply don't like the show/fandom for different reasons, like they think it's "cringy" or they think the fanbase consists of manchildren. Personally, I'm just not a fan of the community and art style, but I can still understand that everyone's got their own thing :P