Honestly, one of the things that pisses me off the most is when people call me "dumb, stupid, retarded" But in the words of Holden Caulfield, "All morons hate it when you call them morons" Take that for what it's worth.
Some people are in fact dumb or stupid. Telling them that they are dumb or stupid in my opinion depends on what they are doing. Sometimes it can be pretty rude, some times it is called for. Just my opinion though. As far as ignorant, it depends on how you define the word. I define ignorance as a lack of knowledge. In that respect we are all ignorant to some degree. For example, someone may make their living as a heart surgeon. Not a field where a lot of stupid people succeed. However, the same person who is a genius heart surgeon may not be able to replace the engine in your car. That doesn't make him dumb, or stupid only ignorant when it comes to auto mechanics. On the same token, you probably aren't gonna want the guy that can remove and replace your engine in 6 hours cracking open your chest. We all have areas of knowledge and expertise based on our experiences and education but there is no one person who truly knows everything. Two good quotes comes to mind: "Ignorance is not knowing any better, stupid is knowing better and proceeding regardless." and "The more I learn, the more I realize that I truly know nothing"