Except that, like OP said, it's a game mode for fun. If someone is forced to play an unfun champ in such a game mode, it's basically two kicks in the nuts for the price of one.
I'm of the opinion that if you're not gonna dodge, then yes, do not "pout like a child that doesn't get it's way". But I also understand that that player is not to blame : He did not pick that champion. I picked Ekko one game on a whim and had a blast with it. Someone on my team however, wasn't much of an Ekko fan and ended having a hard and overall boring time. He didn't go full rage mode but he was definitely salty about the pick. I never got mad at him : I picked this champ, not him.
The moment you pick a champ in All-for-One, you have to be ready for the event where people do not like your pick. This is not a normal game where your team is mad for you going Taric top. This is a game where not only your team has to see you go Taric top, they're also forced to play Taric unless they dodge.
I'm not saying you can't play certain champ. But before locking in something weird, it won't hurt anyone to ask "Is someone strongly against me picking Taric?"