Sorry but the correct grammar would be to say 'You, sir, obviously don't get sarcasm.'

Or if you were to insist on putting emphasis on the word 'obviously' you could instead use italics or perhaps wait until the end of your sentence to insult is lack of comprehension.

Such as; 'You, sir, don't get sarcasm, obviously.'

The English language, like everything else in this world, is only worth the effort if you are going to do it properly.