Do you agree that any of the following is true? -
Ooooh, I like quizzes!
a) One can play team sports badly.
Yep.
b) One can play wargames badly.
Yep.
Yep.
Just, y'know, wondering. . .
Always glad to be of service!
Why would I agree with a, b, and c and yet claim that one can't roleplay badly? Well, there are a few reasons. One is that not everything that can be done is something that can be done badly.
Another is that a and b are games with winners and losers. RPGs don't have winners and losers. PCs explore the world, interact with NPCs and monsters, and generally do stuff, but there aren't any winners or losers.
RPGs have several differences with team sports. For one thing, playing an RPG isn't a team sport in the sense two teams battle it out. If the DM is constantly kills the PCs, it isn't that the DM is a better player than PCs, it's that the DM isn't properly challenging the PCs.
Wargames are in a similar position, they're all about winning and losing. Being good at a wargame means winning regularly.
Also, in an RPG, Bob could roleplay in way that that Susie thinks is bad, but Bob could still overcome the challenges that the DM puts in front of the party. In fact, that's an important part of the chess puzzle example above. The 5 Int barbarian who's player is good at chess can roleplay "badly" but still over come the challenge. In fact, it's almost a prerequisite. In a wargame, for example, if someone wins despite playing badly it's usually because of luck, not because of out-of-game skill.
Acting, of course, isn't about winning or losing at. So why do I think that it is possible to act badly? Because the goal in acting is to communicate something to an audience. RPGs don't usually have an audience. And when they do, it's certainly not a requirement. People don't pay money do see a gamer roleplay. If anything, gamers pay for the opportunity to roleplay. The only other people typically around are other participants. So, a person acts badly if they don't communicate something to an audience through acting. Gamers, however, don't have to roleplay to convey information. In fact, often information is conveyed through other means.
I think that what happens is that someone just gets annoyed with someone else and rather than simply acknowledging that something annoys them irrationally, they say the other person is a bad roleplayer.