Is it good that my mom is really, really honest?

It's good that she is honest with you. Some people are brutally honest though and can hurt other people's feelings. Instead of telling your sister that she looks like a clown, she can tell her that it doesn't "look natural". It also depends on around who she is saying these things, if it's just you and her (maybe another family member) it's okay but if it's around other people that's just mean.