>>282128497
Jimiko implies that they're literally just average looking in-universe. Not just that they have "normal" and calm character designs, but also that they're likely to pass as background characters.
Plain kind of means the same but almost feels like an insult.
On the other hand, the girl at the right in OP's picture is clearly meant to be a cute girl that would be popular in the class and have a lot of admirers, but would turn down most people that confess to her. The girl to the left looks like a middle schooler and would be everyone's crush in middle school.