So first we have to establish who is actually common.
Rule 1. It's not me
Rule 2. Tick off a list of looks and behaviours that are ' common'
In rule 2, though, which and how many attributes need to be counted, one, three, ten?
Also do any 'posh' attributes counteract any 'common' ones? ( if you had a Boris J accent, but revelled in ignorance for example, would the accent trump the ignorance, swearing, drunkeness etc?)
Also does each attribute have the same weighting? We could have 'common TopTrumps!
Then, once one has established that an individual is.' Common' what are we to do with this information? Feel superior, tell them off, move away ,join in?
This is not an innocent post, OP, it's petty and unkind . By all means don't have a slogan tattooed on your face whilst smoking and screaming obscenities at your kids. But name and avoid poor behaviours rather than label someone as common for liking salad cream.