A mild crush is when you have a crush, but it’s not as severe or serious as a full on crush.
You still like them, but you are ok with being friends or just strangers.
Another reason for a mild crush is that you know that you’ll probably never be able to be in a relationship with them so you lower the bar and don’t expect anything serious to happen between you and them.
“I just have a mild crush on them, it’s not that serious. I’ll probably never ever be dating them anyways.”