A mini lol is a laugh that is not quite 'lol', but is still technically a 'laugh out loud'. It is in between ':|' and 'lol' on the lolage spectrum. It's a quiet laugh, the real-life equivalent of 'Heh'.
Person 1: Your mum is fat.
Person 2: Well you're gay.
Person 1: Man, I was just joking with you!
Person 2: ...mini lol?