To my mother and most of her friends there was nothing good about being "posh".
This may well be because the Glasgow area regarded posh people as people who were trying hard to appear better off than they were. To give an example: if they bought anything in Woolworths, they would tell you they had found it in Lewis's household basement or some similar more acceptable shop, even although we all knew that Lewis's didn't stock it, so they must either have bought it in Woolies or The Barras where they pretended never to set foot.
This may be an exclusively Scottish understanding of "posh" also called "Lah-di-dah".
My mother and aunts would never in any circumstances whatsoever have described anyone or anything as "common".
It was common to describe something as common, and even worse to use the word of a person. I have not the faintest recollection of how they conveyed this message to us children, but convey it they did, without using the word "common".
I have an idea it was done by saying that "nice people don't do this, that or the other thing", but how they conveyed the idea that something was posh, I cannot tell you.