Yesterday I saw a Startrek TNG episode that had a Mark Twain character, and he seemed annoying too. So I thought, "I need to find proof that Mark Twain is annoying."
Some writings I found online were long-winded, such as this dialogue that would be severely edited or deleted if it was written today:
"Never has been sane an hour since. But he only gets bad when that time of year comes round. Then we begin to drop in here, three days before she's due, to encourage him up, and ask if he's heard from her, and Saturday we all come and fix up the house with flowers, and get everything ready for a dance. We've done it every year for nineteen years. The first Saturday there was twenty-seven of us, without counting the girls; there's only three of us now, and the girls are gone. We drug him to sleep, or he would go wild; then he's all right for another year--thinks she's with him till the last three or four days come round; then he begins to look for her, and gets out his poor old letter, and we come and ask him to read it to us. Lord, she was a darling!"
(from The Californian's Tale)
However, some of it was sort of funny, such as this excerpt that I saw at a Twain Quotes site:
"You may say a cat uses good grammar. Well, a cat does -- but you let a cat get excited once; you let a cat get to pulling fur with another cat on a shed, nights, and you'll hear grammar that will give you the lockjaw. Ignorant people think it's the noise which fighting cats make that is so aggravating, but it ain't so; it's the sickening grammar they use."
(from A Tramp Abroad)
So it's either that the actors are portraying him in an annoying way, or he really was annoying, and they're correctly interpreting his true personality. If he was really like that, I guess he's the type I would want to avoid at a cocktail party.