Is it true that only ugly women wear makeup?

Not at all. I've seen plenty of plain or below average attractive women never wear makeup, as well as gorgeous natural beauties wearing makeup nearly everyday.
Women who enjoy using makeup wear it. That's what it comes down to the most.

Even some dudes wear makeup..

Where’d you get that idea? All the cosmetics ads feature attractive women. They’re what sell.

I personally choose not to wear makeup (I'm one of society's Plain Janes) but it makes women feel good about themselves so they should be allowed to wear whatever they wish.

Do you find every single woman you've ever seen on tv or in a magazine ugly? Then...no.
I hope you're really young to ask such a foolish question.
Also: many ugly women don't wear make up. I hope your mind is blown

No, anyone who wants to wear it, wears it.

Almost all pretty girls also wear it.

This is entirely subjective.

Beauty is your health and features. Makeup is an aspect of style. It's like jewelry, tattoos or haircuts. Beautiful and ugly women who understand makeup wear it to create various effects.

Oh not at all.. for example my sister is unbelievably beautiful without makeup but still decides to wear it anyway I have no idea why when she’s drop dead gorgeous.

Nearly every woman wants to be pretty... for some reason. I put zero effort in my appearance.

Errr no.

It’s more just a cultural thing I assume.

They only wear it to hide the insecurity.

Nope all women do ugly, average and gorgeous.