No such thing as bad publicity.
I've been in a similar situation. I don't think it can really hurt you at all.
Either the ex is going to build you up, which makes your girl appreciate you even more.
Or else the ex is going to tear you down, which makes your girl test you more, while at the same time she is going to feel like, "Wow, he really affected her for her to have such strong hostile emotions toward him"... which will make her fall for you harder.
The best thing you can do is just laugh about it: "Oh, what bad shit is she saying about me now, lol?"
Then be prepared for any and every criticism:
- "She says you're bad in bed." --> "Oh really? She had plenty of orgasms though!" --> "She says you don't do enough foreplay." --> "Yeah, okay. Well I was always pretty busy when she came over."
- "She says she doesn't like your walk." --> "How does she think I should walk? Like this?" [some silly walk]
- "She says you don't make very much money." --> "Well why the heck was she dating me in the first place? Fell on hard times?"
You can really have a lot of fun with it if you want to. Just have her tell you your ex's complaints about you. She'll love telling you because it's gossip and because she'll want to see how you react. And you get some great intel into what girls say about you behind your back, while getting to have fun tearing up your ex's ridiculous statements without her being there to try to frame them how she wants.
Chase