A 6 year old should be able to wipe themselves unless they have some disability or are autistic.
When I was 6 I could wipe myself but sometimes I forgot to wipe or didn't wipe up good enough which is normal for kids.
If your 6 year old is not wiping themselves properly then you may need to help them and insist that they wipe themselves better.
Some kids just don't wipe as good or want to learn but you need to teach them.
My 6 year old son also sometimes does not wipe himself or wipes himself poorly and so I can sometimes see poop stains on his pants or even smell it.
Then he is stubborn about wanting to change his underwear or pants and I always make him shower and change.
I sometimes tell him that I should put him back in diapers if he's not gonna wipe up after himself but of course I'm joking.
Still when I tell him that it motivates him to go take a shower.