My friend jokes too much?

For example, I wanted to do karaoke one night because I thought it would be fun. She said, "Well it's not like you can sing." She says lots of other similar stuff to that too. I get one joke but she always does it, and sometimes it lowers my self esteem when she says things like "yeah, you're pretty. Pretty ugly." or "your voice sounds nasty. I think i'll buy some earplugs today at CVS." I don't want to say anything about it because I don't want to seem sensitive or anything. The thing is, I can't tell when she's joking or not and some of the things just plain make me feel bad even though she's joking. How do I confront her about it (I already told her, but she doesn't stop. I wonder if she feels good when she puts me down because she has to be aware of it)? Should I even be friends with her?