I would say it, when the t-shirt doesn't bother/disturb me.
Depending on the intonation it can be polite "no", or that
it's ok for me to wear that, though I don't find it particular,
or just don't have strength to say anything else
I would say that depending on the intonation, "może być" goes from polite "no" to "pretty nice". I would also say something like
-You think this shirt will do? (Myslisz, ze może być ta koszula?)
-Definitely! (zdecydowanie może być)