Panel 1: Because it's a hot-ass June day, we find Elly in shorts as she peels the spuds. Since Mike is a disgusting little snot with no damned idea that other people's feelings can matter to him, he laughs at this. When she asks why he's laughing, he states that he doesn't see her in shorts very often.
Panel 2: When he continues laughing at her like the hateful piece of garbage he is, she unhinges her jaw and asks "What's wrong?"
Panel 3: When she explains that she put them on because it's hot in there, he says he has no problem with that.
Panel 4: Since the only thing in the damned world he cares about is how things might affect him, he proves himself a liar by telling her not to go outside. Since she's forgotten that he's a vain mongrel who thinks that the worst thing ever is getting razzed by boys who aren't really his friends to begin with, this leaves her gobsmacked.
Summary: The right reaction would be to defend herself by means of counter-attack and exclaim that the reason she looks 'stoopid' in shorts is because she had to squeeze him out of her pelvis. We all know the stupid thing Elly does. We also know that it's the first part of a very stupid arc about how Mike thinks. Hint: Liz got the fucked-up issues with her physical appearance. He got the whining about how he's the victim of all victims, surpassing all others.