It depends on how it makes you feel. Explain to them that you dont like it. However if they are just joking you need to chill out and take it as a joke.

Yes. You do not seem to be picking up on social signals.

anglian

No, it isn't nessicarily bad, but it's not good, either. You see, if one of your friends are suddenly acting mean, rude, or upset, that generally means that they are having issues at the moment. The issues could be with school, friends, money, relationships, or just life in general. If they act like this, perhaps it would be best to ask them about their feelings, try to find out what is upsetting them. Once that's accomplished, you can try to help them in any way you can. This advice is coming from a girl with a lot of experience with this. :)

Everyone has a bit mean in them. She probably isn't trying to be mean, but trying to have a good time. When she's nice, try to read her expression. Is she really nice or is her smile a phony? Also, tell her that you get sort of hurt when she laughs at you when you fail at something. If she is the girl you think she is, then she'll apologize and try not to do it anymore. She also may try to explain, and in that case, hear her out.

This is just my opinion.