We had a 100 person wedding. Friends, close family, and Aunts/Uncles (no cousins, extended relatives). There definitely were people interested in giving us gifts even though they weren't invited. I told them basically the same thing as this card. It was annoying having to field those requests at the same time as prepping for the wedding, so I could see why someone would send this card preemptively.
I feel like it would only be trashy if you were really expecting money from these people.
You didn't do anything wrong. We did a small wedding too and it was infinitely better than anything I could've hoped for with more people and that's what really matters