(What follows are generalizations that of course have exceptions.) For the record, this is likely less true in Barcelona than in most parts of the US. While a supermarket cashier isn't seen as a good job in Spain, people also care much, much less about having a "good" job, and much more about their quality of life outside of work. Also contributing to this is the fact that the difference in social respect and overall quality of life between, say, a supermarket cashier and an engineer is vastly smaller in Spain than in San Francisco.

Anecdote: my wife is from Barcelona, and is currently finishing her PhD in a technical field. We're in the US at the moment and she could likely get a well-compensated job after graduation if she wanted, but is considering applying to be a barista at Starbucks or similar instead. It's not that she particularly dislikes what she studied -- just that she doesn't particularly care about work life one way or the other at all. I've observed this attitude a lot with Spanish friends.