There are some interesting angles in there to think about. One is that it's just a simple request based on a consideration for others and that, even though the request seems silly, so is being offended by it.
Thinking about how that translates into adult life. We live in a...not a poor area per se, but definitely blue collar 'hood, and I've thought about what it looks like if we ever bought a baller car and I don't think that's a wise choice to make yourself standout in certain have/have-not aspects. Also used to drive to Marysville frequently and would drive past this little group of a half dozen homes that were super run down and then packed right in them was a brand new two story stucco house that stood out like a sore thumb, like someone trying to be king of the scrubs. It looked douchey.
At the same time, conforming for pretty much no reason is silly, especially over something like lunch, though lunch is a bigger deal in the playground world and jealousy can easily ruin friendships, which a kid would have a really hard time wrapping their head around. Buuuut, fancy lunches can also win friends through food swaps and sharing. Any which way, the teacher shouldn't try to mess with things without an actual problem, imo. Kids at school learn just as much about their subjects as they do social dynamics.