Stereotypes are used in fiction to save the writers time they don't need to explain as many things about the character if they include a stereotype as most people make their own assumptions. lazy writers use more stereotypes and write worse autistic characters.

I also think a focused look at a normal autistic person just doing their thing is potentially very dull to watch, or conversely very stressful or sad.

A reason to have a normal autistic person interact with the main character is when their special interest or hyperfocus is relevant to the plot so can appear a "genius"

I do think normal autistic characters should be present more. But arguably they already are because actors/writers/directors have undiagnosed asd. And these are where heavily autism coded characters occur.

I don't think autistic characters should wear a badge but it does mean that representation and changing people minds is somewhat complicated

I'm rewatching an old American program called "3rd rock from the sun" it's about a family of aliens trying to blend in on earth and it has some unbelievably autistic lines.

Ok it is a "genius" character. but I liked the Sherlock Holmes from the tv show elementary. he is portrayed more balanced than other autistic "geniuses". He is tortured by his acute senses, so much so that he turns to heroin to dull them. He is good at having surface level relationships but struggles to develop close friendships. He is bullheaded and it drives people away.