Yeah, I've heard upper middle class types shit on people for wasting their money on X, when X is something they think is tacky, or comes from a big box store they hold in disdain, but if it's something sleek and classy, well, that's different, right?
My take is that most people feel a need to acquire (or "consume") things in order to feel happy. Maybe consumerism is this impulse taken to unhealthy extremes, where people buy goods and services not because they genuinely want them, but because they feel pressured to do so. Keeping up with the Joneses.
If I remember right it was Erich Fromm who suggested there are two ways of living in the world - there's "being" and "having". Maybe consumerism is the latter taken too far, so that your sense of identity and self worth comes from what you own, rather than the kind of person you are.