He walks the line a little bit, dirty though is much too strong a word. That makes it appear as if he does illegal things in the cage, he doesn't. He does kind of bush league things a lot. Hand on the face/throat, attacking the front of the knee, chin in the cheat, headbutting, those aren't dirty, but it's also not exactly the cleanest fighting in the UFC. But this is fighting, there is no reason to flip that a guy is doing things that might not be the most honorable in the cage.
Think. It Ain't Illegal Yet
"Discipline Hurts Less Than Regret"