Paintings were often commissioned works, and like portrait photos now, people want to look their best in them. People put on make up and such to hide pimples and such now, but then could also have the painter help neglect to paint in the unsightly details.
In Act 3 of Henry V, Shakespeare writes about it with "His face is all bubukles and whelks and knobs and flames o' fire", so there is some evidence of it being extant. The oldest example I'm aware of is the Ebers Papyrus, which is a medical document from circa 1550BC, and it mentions using sulfur for helping to clear up skin irritations (something that is still done today).
This topic has actually come up before:
How come we never see historical people depicted with acne? by /u/Bethamphetamine, /u/soggyindo, and /u/Quietuus
Was acne so widespread in the past as it is today? by /u/NotHereToArgue and /u/estherke
Was acne common in teenagers thousands of years ago? by /u/c0mm0n3r
Was acne more or less common in ancient civilizations, or even a few hundred years ago? by /u/phan7om
In the past, what was done about acne? How was being spotty perceived in society and were there any treatments? by /u/cdesmoulins
Did people have bad skin (acne) as often as they do now before this century? Why do I never notice it in old photographs from the 1800s? by /u/colevintage
Was painful Acne ever an issue in WW2? If I slept in the dirt for weeks between ages 17-22, I'd have breakouts constantly. Is Hollywood's portrayal of dirty--yet unblemished--soldiers accurate, or do we have photos, documentation to show otherwise? by /u/Georgy_K_Zhukov, /u/Charrikayu, and /u/UnparaIleled