Ironically Karin a better catch for him at this time even though she's very similar person to what Mona was in the past (I root for Asahi though I know she's a losing heroine). Medaka is way too passive therefore it's going to take an aggressive woman to get him to react, most people are like this because of their fear of rejection. When it comes to first relations, usually the first person to confess or confirm the attraction usually has a chance even though they may not be the first or even the best choice, just simply the safest and easiest. In fact all the main reactions Medaka had towards Mona was because she got in his personal space trying to provoke some form of sexual tension or attraction, right now she's lost her confidence while this girl. Ironically her warning/threatening Karin to keep her distance is hypocritical as this was the same thing she did when she had no interest in him neither.

she did have a point tho, mona has no grounds to tell her to back off