Edit: I mean I'm aware of 'welfare queen' as an insult but I've never seen 'welfare mom' have that same meaning, or anything other than 'mom on welfare'. And when you're talking about socioeconomic status 'welfare' doesn't have to be quite literal, just signifying a level of income.
This is used in race studies but also class studies for instance 'chav' in the UK and your use works much the same way. It means you are identifying someone in a way that it is most likely they would not identify themselves.
You are matching your perception of someone with your pre-existing notions of them - typifying them in a sense. There are so many things to consider, really.
If you see someone in a Lamborghini, you might assume they are rich. But they may in fact be massively in debt. Therefore the analysis becomes a social one - about there access to funds or cars and not necessarily related to their economic status.
If you see a mother with a phone you think she couldn't / shouldn't be able to afford (I realise they used this example as a past belief but may as well use it). The phone could have been a gift, it could have been won - there are so many factors that you are reducing to one.
The point of identifying a trend is to reduce the factors. If there isn't in fact a trend, that's when you look very closely at the individual person, or that's the point where you embarrassedly drop the subject because you made a false correlation.
Side note: I consider you rich if you can make payments on expensive-enough things, even if your debt is greater than your assets.
If "welfare mom" was an insult, it would probably refer to that.
You do not get it boss. He did not say he was wrong about assuming their social status, but he was in mistake for assuming they had better spend their money else where other than on a smartphone. For all we know he still looks at those people as "welfare moms."