Really good question :) "ゆる" comes from "ゆるい" which means ”loose” or "casual" The opposite of "ゆるい" is ”きつい” which means "strict" or "tight" or "formal". Think about their looking. They don't look like formal, classy, and sophisticated looking. They look more like casual, harmless, and friendly looking. That's the idea of "ゆる" or "ゆるい"!!