I'm thinking of hiding my girlfriend's hoodie
Why would you hide her hoodie?
The hoodie is ugly and she wears it too often
Taking property that does not belong to you is stealing. If you do not like the hoodie, you could try to buy your girlfriend a new hoodie so that she will wear that one instead, or be honest with your girlfriend about how you feel about the hoodie.
But I really don't like it.
You don't really get to choose what your girlfriend wears though. If she likes it then you should let her wear it.