I thought Max was a really good character for exactly the purpose of explaining AS to people in a simple way. He's definitely (and necessarily) a composite character. I was thinking about this when I watched the movie. I mean, the way he gets upset about littering. It's a combination of very black and white thinking, meltdowns, near obsession, and literal-mindedness, all directed at one thing. Traits are not always quite that obvious in real life. Some people are more intellectual about their interests, some more emotional, some kind of neither, depending on their personality. So he sort of describes everybody and nobody.
And Max's special interests and routines were kind of condensed into one and the same. I mean, he liked chocolate so he ate chocolate a lot and that was it. If it was just part of my routine, I can see having a comfort food. But if it were like a special interest, I'd be researching all about the origins, history, production, etc. of chocolate and telling everyone about it.
Max is a very uncomplicated guy in a lot of ways. He has childish interests, expresses himself like a child. But a more intelligent, nuanced (real) person with AS could easily experience all the difficulties Max has. That's the thing I'm afraid people don't get. I think the movie does a good job at showing a lot of hints that this is only a little animated world and the characters are only representations, but some people will not be able to see that there's a bigger reality behind it because they aren't really understanding AS. If you do know about it and watch the movie, though, you can see the filmmakers were aware they're providing an incomplete picture and try to throw irony and little damning details in. So overall I like the character of Max and the way he interacts with the other characters.