This is certainly an interesting development... Mei seems like she would have been content to just casually 'use' Yuzu with no thought of the long term issues. Then Matsuri comes along and flat out tells her she's just using Yuzu, and that she should just piss off, because she doesn't really care about here anyway.
Matsuri has effectively delivered a declaration to Mei: 'you don't really care about her, so what's the point in fighting over Yuzu, now leave.' She expected that Mei would react from a position of anger, that she'd react to the insult to herself and and simply hit her. Instead, Mei reacts to the declaration with a declaration of her own: 'No, Yuzu is mine and I'm not giving her up to you or anyone else.' Hopefully, this means that Mei finally got the proverbial kick in the ass she needed and will start taking things seriously- that she'll finally by honest with both herself and Yuzu.
I'm not sure what the heck Matsuri is planning... She's like 15, and she behaves like she's some kind of diabolical mastermind and manipulator. I doubt Yuzu is so gullible as to think that there's something between the two of them, so the pictures could only serve to possibly humiliate Mei by publicly outing her. And even then, the only person she shows any level of weakness around is Yuzu, so shaming her would likely only drive her back into Yuzu's arms.
Rather than keep her as a harlot and a villain, it would be more interesting if the story actually addressed why Matsuri is so messed up in the first place. She clearly appears to have been emotionally/sexually damaged at some point in her life, and is unable to form normal relationships with people- hence her casual admittance of 'compensated dating' to Yuzu. She clearly needs help that she's not getting.