Exactly. I do some things better than any character I have ever made does and my characters do lots of things I could never do nearly as well. I had a character with a +18 diplomacy. I have no idea what he would say or do to get reactions like that with anyone he talks to.
You don't have to be charismatic to play a character with +18 diplomacy (though admittedly, it helps).
You do have to make a brave attempt at playing the character though, whether you are personally charismatic or not.
The dice is there to help me as the DM interpret what you do in the best possible light. It's isn't there to replace the role play itself.
I can make vague statements. "I want to improve the dwarfs opinion of my elf and stop associating me with all the other elves he hates because he is a racist. Yeah I would have no idea on that one."
At the very least you ought to try saying, "I'm not like the other elves. You'd like me if you'd get to know me. Maybe its time for you to show how noble you are and judge me for who I am, not for who your racisms tells you to believe I am."
No, maybe that's not a very good thing to say. I don't know. It's quite possible that you say that to the dwarf and he's insulted and pulls his axe on you. Or, maybe you force him to rethink his actions. I don't know.
And that is why we roll the dice. And, because your character has a +18 bonus to diplomacy, chances are your brave, bold, frank attempt is going to work at least a little. On the other hand, were you playing a character with a 4 charisma, chances are it won't work. But the important thing is that, either way, you the player make an attempt to role play it out, define the character, play the game, and interact with the game world.
Saying, "I want to make a diplomacy check to improve the dwarves mood" is interacting with the rules, and not the game world. If your proposition doesn't involve your character interacting with the dwarf, then I'm not required to respond to it as if it happens.
I tell the racist to F-off and consider hitting him over the head with the sword I am carrying.
And if that is how you choose to play your character, then it doesn't matter much what diplomacy skill you have. Nonetheless, it becomes who your character is. Your actions as a player define the character.