Mina is ostensibly timid and shy mostly according to Ian who doesn't really know her all that well because he doesn't seem to have much interest in actually listening to what a girl may be saying.Actually mina getting mingled with her ex's bff is the least of the inconsistencies in her case, you forgot to mention that bucket list of hers, it came out of no fucking where, at one point she is so timid and shy that we have to gain bi curious points with her to get a possible future threesome, and then suddenly she through a bucket list full of depraved and "out of question" dirty things (for someone like her).
The bucket list is something Mina simply didn't share with anyone (or at least not with her now ex-boyfriend) but we also know that she watches porn (again, weirding Ian out because, you know, a girl watching porn, inconceivable) so it's not like her interest in sex comes from nowhere.
Also, nothing really odd or timid and shy about developing one's interest in the same sex in early twenties.