unless youre talking about fire retardants, the timing of engines, or other situations where a thing is being actively retarded, dont use the r slur. Applied to people, it is a slur.
edit; y'all wanna down vote and support the usage of a slur, even when presented with reputable source evidence explaining why it is a slur. grow tf up.
BTW. When the DSM-5 talks about "a strong sense of justice" this is what it talking about. It's not the sense of right and wrong that's diagnosing, it's missing the obvious fact that this joke came after what was probably 10-30 minutes of the comedian assessing how many people in the room were ADHD like him, then building up context that would allow the joke to land without hurting anyone's feelings.