It functionally should not be considered to be a slur. Almost nobody who uses that word is going to tell it to an actually mentally disabled person. And you should always be able to make fun of the attribute of intelligence because it directly correlates to competence
The reasons why the f slur or n word (or t word for that matter) are bad is because they assume that just because you are this quality that only changes one thing about you that you automatically are worse. Being gay or black or trans does not affect your ability to be smart... but being an r word does