I refuse to clean up my roommate's cat's shit.
Is this something she's asking you to do?
No, she just complains about me not helping out.
I understand you not wanting to have to deal with that.  It's important that somebody takes care of it though, as leaving animal feces around is very unhealthy.  Make sure it gets taken care of!
She's going to have to deal with it herself, though she'll still complain. I'm too busy with my own shit.
So long as you know she'll take care of it, with complaints are not, it's not your responsibility. Have you talked to her about balancing out household tasks? Everyone should do their part, but looking after her cat isn't your responsibility.