I wiped his mouth off with a napkin
Is there something about him that made you wipe his face with a napkin?
He was eating from my plate and made this ghastly face at me.
If the person that you wiped up is your child then it is perfectly acceptable for you to wipe his mouth off with a napkin. It's expected that parents clean up after their young kids and to keep them clean.
Why? I was only stating what is going on and I didn't do it in front of other people.
Well have you ever seen a child with a pudding moustache, not the greatest look. Kits are messy and they wipe their hands and faces on all kinds of things to you better keep them clean.