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).