I didn't take that as the mess was Ryan's, only that he thought it would be Pam's job to clean it up.

Based on my experience in various offices, I think it's pretty clear that was the sort of mess that was probably one person (who I think we can't name based on what we saw) who made an already messy microwave really messy. My guess would be that whoever it is would try their best to ignore the whole thing, lest they attract attention to themselves.