Not sure if he's dirty in the sense that his techniques are illegal, but they are definitely frowned upon.
Headbutting abdomen of opponent, stretching fingers towards opponent's eyes to discourage forward movement... no good coach tells you to do those things and a supposedly super-talented fighter who usually enjoys a size advantage should not need to use such techniques.
Sure it messes with opponents mentally and is part of the formula that wins his fights, but I find it unnecessary. Spinning elbows are fine, the one he dropped Bonnar with was blatantly illegal but my memory may be wrong. Either way, it wasn't called into question.