Basically, the "be careful who you call ugly in middle school" is a meme trend, that of which is where you take two people/characters/etc. that are often joked about as looking "similar", but with the one being seen as more "attractive" than the other. Hence, with this meme, you have an "unattractive" shot of Toon Link (from 'The Wind Waker') paired with a more "attractive" image of Breath of the Wild Link. However, it can also be used with the same person/character/etc., just with different pictures of them (again, with the one imagine being seen as more "attractive" than the other). Now, there are other ways in which this trend is carried out, such as the change being worse than expected (instead of there being a "glow-up"), for a surprise factor.
Hope that helps!
