It's not so much that the hairstyle is seen as "bad", but more like it's the representation of "childish" and/or a "screwup by the hairdresser".
The bob cuts are normally used by elementary schoolers and by people that don't pay a single moment of attention to fashion or looking cute, hence why most characters that use the bob cuts are either lolis or background characters.
Sure enough, you CAN make the bob cut look, not only good, but damn impressive, but it needs effort, the right body type (real or simulated) and coordination on the clothes. So if you get a surprise bob cut from your sister that knows jackshit about cutting hair and your previous style had nothing to do with bob cuts, then you're gonna be mortified and think you look horrible.
So yeah, poor Hina.