I think what complicates things is there is a LOT of vagueness between the two. You only get what Lexi thinks in the argument, and what she tells you about the moment if MC is on her route. Erynn's retelling doesn't have the same details post-MC leaving that Lexi does, but as Erynn wasn't there that's not surprising.
If Lexi really had zero connection with MC, then why stop? Why would she feel he needed to be consoled, wished happy birthday, and then kiss MC on the cheek? Yeah I'm reading between the lines, but it just doesn't strike me as something someone with zero interest/connection would do.
In the flashback, the MC never says it's his birthday and Erynn never says it, so how does Lexi know? She can't just have guessed, and there's no indication they hung out earlier in the day based on Emily's conversation with the MC when he goes over there. If MC and Lexi had no connection at all, why would she even know his birthdate? Again, some of the pieces just don't fully add up in my mind.
I don't think it was a mutual connection, but I just get the feeling that Lexi may have had some interest in MC, and comes across as very shy about it.