There's nothing basically wrong with it, it's just that people who try to make their slow car look fast with rice also tend to genuinely believe their slow car is fast, and want to show off how fast it is. That means they tend to do dumb shit on the road, always talk about how much faster and better their car is than everyone elses, get really upset if anyone does engage with their shit and show them how slow their car actually is, and generally just be pretty damn annoying to be around.

Not everyone who rices out their car is like that, some just genuinely like the look, but a lot of them are and that's the first thing people tend to think of. That's where the hate comes from.