Long answer but here's my two cents on it:

It would be an insult to say Keiko was mentally ill or on the spectrum. I feel the author's intent was to show how people are able to lead any type of life they want if they pay no regard to adhering to the pressure of societal expectations.

It raises a lot of questions about how these standards came to be and why people feel the pressure to follow them. We aren't here on this Earth for long, and are only given a single chance at life, so why can't people choose how they want to spend their time. Who cares what anyone else thinks or why should we care how another person lives there life if it doesn't affect ourselves in any way?

Why do we need to be married and have children at a certain age?

What does it mean to have a "real" job?

Why do we need to regularly hang out with people?

Why do we always have to be dating or be in a relationship?

Who established these rules and why are they considered the blueprint and dictating they if they are not to be followed- you would fail to live a meaningful life?

Keiko was perfectly content and happy with her life at the convenience store. She found her purpose and didn't let others define it for her. Most people fill all of these expectations society doles out- or at least try to- and are still dissatisfied and lost. One of the reasons for this is that they do all these things to make others content or think think these expectations draft the answer key to their own happiness. Yet, nothing is guaranteed and things don't work out the way you want them to.

Shiraha's character served as a metaphor for society. He would openly as Keiko questions that would highlight her differences from her peers. People always have to put a label on things they don't understand when how others choose to live their lives is a concept that doesn't need to be understood by you and only matters to the person making their own decisions. That doesn't mean Keiko has to be autistic.

It's rare to see someone passionate about what is considered a "simple" job when most people complain about work, their kids, marriages, etc. Keiko was able to figure what she liked right away and didn't have any reservations about the decisions she made. Most of us wish we could go back in time and change things if we could. It was almost like she couldn't let herself think any other way.

I thought it was a refreshing take on expectations of women in society as well as how simplistic the plot was about someone who dedicated themselves to their passion in life no matter how meaningless or bizarre it may come across to others.

TLDR; just because someone chooses to live their life differently than the usual norm of society doesn't mean they to be labeled as mentally ill.