Lara Jean might resent Peter for his popularity, but she acknowledges that at least in some ways, he deserves to be so popular—he’s a genuinely nice kid, if a bit entitled. This shows that she knows him on a deeper level, which makes sense given that the two were part of the same friend group at this time. So, her assessment of him, while written in an emotional and immature tone, is possibly more balanced than it would be otherwise. Her comment that she’ll catch and STD from Peter, however, doesn’t seem to be based on fact—rather, it seems like a false accusation rooted in Lara Jean’s jealousy of Peter and Genevieve’s relationship.