There are a couple of studies on this, and the findings are admittedly kind of scattered, but here are some key points.
One study suggests as much as 50% of people on the spectrum have at least four (or more) comorbid conditions (a vast variety of things, e.g. ADD, intellectual disabilities etc).
Another study found that 95% of children with ASD had comorbidities. Again, the rain man trope is rather strongly rooted in our sociocultural reality tunnel.
I've met a lot of people on the spectrum in my life (about 1% of the population are diagnosed, after all), and almost all of them have also had either ADD, intellectual disabilities, medical disabilities like Crohn's disease, Ehlers-Danlos syndrome, PCOS, hormonal or chromosomal disorders and/or other psychological conditions like schizophrenia or other schizotypal disorders BPD, bipolar disorder etc.
The fact is that a Sheldon Cooper is kind of a unicorn among autistics.
But truthfully, my observation that most autistics are below average intelligence is almost entirely anecdotal (yet with a rather sizeable sample pool), because admittedly I'm struggling to find any hard data on it.
Some figures I've seen say 32% have below 70, 25% have 70-84, 40% have 85-115, with only about 3% being above 115.
But here's at least one study finding that 55% of autistic kids have an intellectual disability (below 70 IQ).
We lost our sphynx very dramatically two weeks ago, and he was only 10 years old with no prior health issues, so I know how it feels.
You're not alone.
Me and my partner are still devastated and find ourselves sobbing uncontrollably every day.
It was like losing a child.
Life is on auto pilot these days.
I hope I'll see a good day again.