Not exactly related, but some time ago I realized it's not ok to make fun of people with physical disabilities but somehow it's totally acceptable to make fun of people with lower IQ.
I think others are confusing your "lower IQ" comment to mean "people with intellectual disabilities", which I don't think you're defending. It's obvious that "stupid" people (those seen as "normal stupid") are one of the last publicly-acceptable targets of ridicule. It's common for even the most well-mannered public personalities to speak derisively and dismissively about the stupidity of others. The most common insult toward someone you disagree with is to call them "stupid" or "an idiot". No one says "retarded" in polite company anymore, but it's essentially the same thing.
Funny that you mention it, but at one point "mentally retarded" was a gentler term for the then rather harsh clinically-used terms "idiot", "moron", and "imbecile". Now these three words are considered much less offensive than their one-time euphemism. What a world.
I don't think it is? Making fun of people that do one stupid action is I think okay, but making fun of people that have a harder life than other people because of how they're born doesn't seem okay to me.
Not OP. FWIW, I think OP isn't referring to individuals with intellectual disabilities; just those with an IQ one standard deviation down from the mean e.g. 85.
In my experience, there's often no backlash against jokes or even serious talking points about "stupid people".