It is important that we have a "good time", If we don't, then our role-playing will start to suffer, and we might end up quitting. But we should not forget that we have an obligation to the people that we are roleplaying with. We need that other player. If we didn't, then we could have simply written a novel instead.
So the people we roleplay with also need to "have a good time."
If you play in an RPG based on say an anime, you want to roleplay against those characters, right?
Is it fun then if the other person can't properly roleplay?
I get what you are saying. If you have a favorite character, you want to roleplay as it, just like a cosplayer wants to cosplay as their favorite characters.
But if you look nothing like the character you cosplay as, this only really affects you as a cosplayer. But just like how "the wrong actor" can ruin a character in a movie, the same goes for an RPG.
If a person can't capture the personality of an already established character, then it isn't fun to play with them.
To wrap things up, I'll end it with this.
If we can have expectations on a roleplayer's ability to form proper sentences, then we can have expectations on them being able to properly portray the characters that they have chosen to play as.