It's down to personal taste.
I like Lady gaga because she's different and breaks the mold, i also thing she is a very talented lady. She is also gaga (a bit crazy), and i like that.
To say you personally don't like Lady gaga, IMO is fine, you're expressing a personal opinion. However, when someone makes a statement rather than expressing a personal opinion saying "she's rubbish", then they need to back that statement up with facts, otherwise people will shoot holes in that statement simply by giving some facts, in this case, the album when platinum, which suggests that a lot of people don't agree that Lady gaga is rubbish.