I suppose its down to your own view on things, I consider it rude and disrespectful and even though I know I have done it in the past, I dont like it and make an effort not to do it
having the card out at side waiting, for me, is fine, if anyone feels the need to prove they are issuing a sanction before the protesters gather, but, again maybe just for me, its akin to kicking a man when he is down. Guy has committed the foul (can argue whether 2nd yellow or red), (am in the red camp), give him a second to get to his feet and treat him like a human being, not some game deer shot for amusement.
I also look at it like warning a player and he starts fiddling with his socks, if we prefer him up and in everyday pose for this, I think it follows suit that in return we do the same with a card. We classify this as the player showing disrespect to us when he does this, so, vice versa I think its disrespectful of us to card him when he is down