This cartoon is based on the idiom, rub it in.
rub it in - if someone rubs it in, they keep talking about something or doing something that makes you upset or embarrassed.
"We all know she made a mistake, but you don't have to rub it in."
"I wanted to rub it in, so I said, "I'll be thinking of you working while I'm on vacation.""
"John failed hix exam, so to rub his nose in it, I put my certificate up on the wall."
