I think my co-worker's an irresponsible asshole
That's harsh. Why do you think your co-worker is an asshole?
It's because his work is a continual let down.
I know that you don't approve of your co-worker's work, but that doesn't make your calling him foul names correct. Deal with his work without characterizing him as bad.
Im right and I know what I'm saying.
If your co-worker or anyone gets wind of you calling him names this may cause an unpleasant work environment. If you think he's struggling with his work why not offer to help teach him how to improve?