I don't think Ndamukong Suh is dirty. He's just a big physical football player that looks to bring the hurt. Yes, there are some plays that people will use to say he's a dirty player, but he's not dirty.
Playing in the trenches is always rough and you have to be tough to be in there every week. He is very talented and has made some questionable plays, but I'm not sure I would say he's dirty. He is close to getting that label though, and once you get it, its going to stick with him through his career.