I'm scruffy (as in jeans and bodywarmer all the time, never smart) but am NT. I used to joke with other mums about how nice they all look compared to me. I don't bother with make up any more, and some of them never leave the house without it.
I am older than most Mums, maybe they just labelled me the old frump - but I socialise with a few of the older ones and we all get on well. I don't think being scruffy is your issue. Nobody avoided playdates, as far as I know.
My son is autistic. You may be right that being 'weird' (which is how we tend to describe his foibles) is what makes people a little unsure. I'd personally go against the general view here and say yes - why not mention it? Lots of people are happy with difference if they can 'label' it - whereas if they just think you're a bit odd they may be nervous about who knows what......?
In what way do you think you are weird? It's often socially - do you struggle with small talk, or to feel you're getting it wrong? If so, then I do think a 'sorry, I'm so rubbish at expressing myself, it's because I'm on the autism spectrum. I always get it wrong, so glad Billy takes after his Dad, ha ha'. Or similar - might just reassure other parents?
There was a boy at school with DS who was so obviously aspergers, and so was his Dad (who did the school runs and joined us in the park afterwards etc). Neither diagnosed, clearly, but I always felt sorry for both as people did avoid the Dad, and the lad struggled a bit with friendships etc. I always felt they dipped out because people just thought they were a bit weird. The boy had a party aged about 9 where his parents had paid for a certain venue, and people all accepted - then on the day less than half actually turned up! People can be horrible.......
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We have never hidden DS's diagnosis (ASD and ADHD) and he doesn't either, although he has friends who are less open. Ideally I wish people could be loud and proud, but I know it's not that easy. Of course you shouldn't have to tell anyone anything, and parents should be far more considerate - but if you feel it would help your DS with friendships then I totally understand why you might want to. And you probably only need to mention it to your son's best friend's Mum and then she can let others know, if you prefer? Good luck anyway, with whatever you decide. I hope it all works out.