Papagenos (and no one else, really) is under any obligation to have to like a character or pretend to like one. However, you also have to be a decent sport about the whole thing. If Papagenos went "Well, that sucks, I don't have any attachment to this character and I think there's too many Fire Emblem characters already, so I'm not that excited. Congrats to their fans, though", we wouldn't be discussing him". Instead, he basically went to an extremely hyperbolic rant that didn't accomplish anything, completely ignored any substance of Byleth because Ignited Stamp bad, and went about the entire thing like Sakurai just announced he was removing Banjo from the game.
If you want a good example of someone clearly dissappointed taking the piss of Byleth in a respectful way, Maximilian's the guy. He didn't even try to hide his disappointment from anyone, but he instead used the situation for comedic purposes and wasn't completely shut off from the idea solely on principle.