She stated that she thinks it's great when 2 girls are close, so it's a possibility. And the fact that she decided to go to an all-girls' school makes you wonder too.
No she's not. She's just not used to seeing very close girls that's why she's like that.
Put her in an all boys school and she'll like it when she sees guyXguy relationships.
She's curious about everything! and I mean everythin! So if she seems like she wants to finger mio, it just means she's curious about mio and forehead.
In the manga she tends to have yuri like fantasies from time to time, so if I had to guess, Mugi is the mostly likely girl in the K-ON universe to be at least bisexual.
I doubt that shes a pure lesbian I think that shes just a bit of a pervert like that blonde from Shuffle who fantasises about EVERYTHING... If Makoto Itou joined K-ON! I reckon she'd be up his trouser leg like a whippet!!