This one is somewhat subjective based on how you define acquaintance.

I'm thinking of my own neighbors, I'm not particularly close to any of them and if I saw them on a flight we'd likely head nod as acquaintances but I can see how sending a drink to them could be perceived as trying to show off even if it wasn't intended that way. A neighborhood acquaintance likely doesn't know how much I travel and doesn't know that most of the time I'm in FC because of free upgrades or earned upgrades -- aka RSUs/PPs/etc.

On the flip side if I saw a close friend (or close acquaintance) on a flight and sent a drink back, it would be welcome because there's a more personal relationship and both parties know that you're just spreading the wealth and helping to save the friend in coach a few bucks on having to buy a drink themselves.