I want to clear things up with my now ex after she had the impression I wanted to cheat on her
What gave her that impression?
I was joking about it and she thought I was serious.
I suppose you're trying to offer her some closure. That's fine as long as it's reciprocal on her behalf.
Do you think it's a bad idea?
No I don't think it is a bad idea. It is fine to be open and honest with people about these things, even if the relationship is already over.