OK, Uggs are not especially attractive. If a girl is wearing uggs, a guy may think she kind of looks dumb. If she's wearing them in the summer time or in the spring with a mini-skirt, most guys will think "Wow, this girl looks kind of stupid."
Yes Uggs are comfortable and if you are wearing them lounging around or indoors, in the winter time, etc. no one is going to care either way. Guys don't really care about a girl's shoes too much anyways.
So it has to do with context. Just like wearing rain boots when it isn't raining out would look silly.