You could use this interchangeably if you're implying that the person being talked about looks like their mother when they get upset in the first sentence.
There is a slight difference here. "You look just like your mother when you get upset." This sentence is clear that the person looks just like their mother when they get upset.
"You are just like your mother when you get upset." This sentence is unclear as to what the person is specifically like when they get upset, it depends on how the mother acts and what you are implying. This sentence is not specific and can mean that the person looks like their mother, sounds like their mother, talks like their mother, or anything else that their mother might be when she's upset.