Mess is good, especially for children. If I changed my 5yo's clothes every time he got a little wet, dirty, etc then he'd never have time to play & have fun. Sure, if we're home I don't let him stay in wet/dirty clothes, but if we're out for a walk and he jumps in a puddle and gets a bit wet or dirty I don't strip him off in the street and make sure he's presentable before we go into the shops for morning tea. Equally I pick him up from Kinder and he might have paint on his clothes (usually not though) and often has sand/dirt/water on his pants (and/or top) and I don't insist on changing him before we do things on the way home. Where would I change him anyway? In the tiny toilet at Kinder? In the car boot? He's 120cm tall and in size 6 clothes, it's not easy to change him and why bother? He's a child and he's allowed to play and enjoy life including the messier side of life!
My dd (8 months old) is a different matter and as she's only little and easy to change and more likely to get covered in food mess, I will change her in public. I take her out in clean clothes (99% of the time) and if she gets very messy I will change her if I know we're going to be out for a while. But if we've just had morning tea at a coffee shop & we're going to grab a couple of things from the supermarket before heading home I wouldn't bother.
So, I think judging a parent for having a child with dirty clothes on a single occasion in public is very judgemental. There are heaps of good excuses and to even suggest that considering it as child abuse is way over the top IMHO.