I would never accept a gift like that on my birthday >=(. But I will accept it anyway and I will hide it from everyone, just because I am such a nice girl, but I wonder if that happens to me, if my boyfriend know me well.
But that people will bullying her like that is actually wrong.
I don't know what kind of situation she has at school, but bully just because of a shirt seems to far for me.
You should talk about this situation with her. Give her a new gift, a gift she might like. But not because you think it's cool. She must say I like it.