It’s possible Susan would feel the heat less if she cut her hair short, but nobody point that out. She’s already touchy about the various ways her long hair could be considered illogical, and she does like it.
Susan nearly said “dress like Diane”, but I thought about it, and no, she wouldn’t. Diane wears skirts, which Susan doesn’t care for, and Diane sometimes wears strapless tops, which Susan also wouldn’t pick. She might choose things that show roughly an equal amount of skin depending on the weather, but she would still be distinguishable from Diane.
One difference between Susan and Diane in this case is that Diane considers what others will think of her outfit beyond “will I feel embarrassed”, and is actively hoping to turn heads. She likes that, she cares about that, and she will put effort into that.
Susan, meanwhile, dresses only for her own satisfaction, but is kept from dressing exactly how she’d like in warmer weather due to feeling self-conscious. In all situations but “it’s hot out”, she doesn’t consider what others think when choosing her outfits.
And before anyone asks, I’m sure some people wear short skirts and small strapless tops purely for their own satisfaction and not to turn heads. I am not stereotyping outfit choices, I’m just talking about things that matter to Susan and Diane.
In addition to not wearing certain outfits, Susan also doesn’t own an enormous hat or parasol for similar reasons. She would totally have one or both of those for dealing with the sun otherwise.