Calling someone common doesn't make you the common one- judgemental, maybe, but not common.
Common isn't having a northern accent (southerners can be equally as common), it isn't having no money, living on a council estate, swearing, being poor or being rude.
Common is when a person looks cheap and tasteless- they might wear something considered tacky like a velour tracksuit, a mini skirt with fishnets, and lots of leopard print, football tops for every occasion, too much makeup, fake tans, footballer type haircuts with patterns shaved in, shirts off at the first sign of the sun in a beer garden. When they lack good table manners, use poor grammar with lots of slang, smoke and drink combined with the above, etc. etc.
It's not one thing in isolation, but a combination of several things.