I don't think I've ever had any complaints from parents about me thoroughly thrashing their children in a card game. Even though I was using Gigas (and, lately, Durant).
I think the worst thing I ever did at league was teaching someone how to play, but I was using Cursegar as my example deck.
"Can I play Trainers from my hand?"
"No."
...Didn't work out very well. I felt really bad. I never did it again.