It's not that the picture itself is too racy; as you say, by Rooster standards it's pretty tame. But it IS suggestive and that's an odd thing to post when Maya has been so worried about anyone finding out she likes girls. (Again, unless it's the MC posting this.)
But her and Josy are friends from school, so friends posting mildly racy pics of each other on social media is not uncommon.
Josy posting the pic takes that problem to a whole new level, because we are explicitly told Maya's dad cannot learn they are roommates. I've been willing to turn a blind eye to Derek's self-incriminating clucks in the past, but if Josy really did post it would be a fatal blow to the notion that Maya's dad is controlling her life. If he doesn't notice something this blatant, how could he possibly have managed to keep the two from communicating over the summer?
That in turn would severely undermine Maya's character, because it would appear as though she put zero effort into remaining in contact with the girl she supposedly loves.
If Josy is the one who posted that then it would be a severe lack of judgement on both their parts to not think that Maya's Dad would be watching their social media pages, but it would likely happen in order to force that confrontation with her Dad. Odd example, but it reminds me of Cool Runnings where Junior doesn't want his Dad finding out he's joined the bobsleigh team but then a picture ends up in the newspaper and that's how his Dad finds out. It's not the same as him sending out that picture, but it's a similar contrivance in order to force a confrontation between the 2 characters.
This could just be a careless moment if Josy is the one who sends it, but I don't think it undermines Maya's character. There are ways her Dad could have stopped them communicating, like taking away her phone, laptop, tablet, whatever she could use to communicate with Josy, maybe even monitoring her phone to see who's calling her, not letting her leave the house by herself, etc.
To be honest though, this is probably one of those things in fictional stories that we're just supposed to take at face value and accept that he somehow kept them apart even if the methods might be hard to discern and even harder to make sense of.
Did Maya, Josy, or Derek ever say why maya’s dad can’t find out she’s living with josy or seeing her?
Not explicitly, but it's heavily implied that he's a bigot who disapproves of her relationship with Josy. He's also holding her tuition hostage on the (likely) condition that she renounce her relationship with Josy, although that is not confirmed but again, implied.
You know how many girls are bi in this game?
By my count, about 8-10.