"Are Autistics generally insecure about their appearance?"
One word answer "No"
Two word answer "Some are"
There are those of us who are prone to obsession, not just those with OCD, and that obsession can be over our appearance. For many that will be borne of the belief that looking a certain way will ease of social difficulties, but most of us find eventually that it makes little difference. That happened to me in my late 20s and I've not looked back since. I've better things to do than coiffing my hair or worrying about what to wear. If it's appropriate for the occasion, that's enough. Hair cut so short I need not fuss, shoes comfortable, clothes practical. My one concession is my preference for T-shirts featuring things I like - SciFi shows/movies, favourite music and others.
Dysmorphia? There is little to support the idea of dysmorphia being significant amongst Autistics, however there is a notably higher proportion of transgender Autistics than in the allistic population. Whilst not dysmorphia in itself it can have a profound effect on how we perceive ourselves and our appearance
Unkempt or dirty Autistics? Either executive function problems, not caring about appearance or a combination of both. Henry Cavendish - a great scientific pioneer and one of the most notable, positively identified Autistics in history, famously wore the same outfit his entire adult life, replacing it with an identical one when it wore out. The only variation was that his coat was sometimes green, sometimes violet.
Our tendency towards anxiety can certainly manifest itself in matters sartorial, particularly when we are insecure, but that often fades with age as it becomes apparent that no matter how much we work on our appearance we still get the same reactions from people.
As with many things we look at through autistic eyes, we see patterns, or we seek to identify aspects of ourselves as autistic rather than our own peculiarities. In this case there appears to be as much variation within us as in allistics.