Do you have any data for this distribution? This is the first time I've heard someone suggest any particular shape to this distribution, much less that it's shaped as you suggest.
I published this paper recently (1) in Biological Psychiatry. I bring it up only because unlike most neuroimaging research, we are able to image children at all levels of disability (since we image them while asleep). We argue that the idea that there is early brain overgrowth in autism followed by a period of regression or normalization of brain volume does not appear to be true, and the apparent differences in cross-sectional research are due to biases in most studies against including harder to image individuals with autism. In any case, intellectual disability is not 90% but it is quite substantial.
I don't know about these precise numbers, but ~57% of autistic people have an <85 IQ (cf. 16% in general population), and somewhere between 25% to 50% are nonverbal. So 90% may be too high, but it seems very likely that a solid majority has profound difficulties in life.
Note that these numbers are for autism diagnoses, not autism cases. Someone with profound difficulties in life is far more likely to wind up with an autism diagnosis than higher-functioning autistics that are more likely to figure out successful adaptations.
1. https://news.ycombinator.com/item?id=25462145