The Underground Man wishes that his constant state of inaction were simply because of laziness. That sure would make things easier. And at least then, he says, he could respect himself.
Also, at least he would have certainty. If someone asked what he was, the answer would be clear: a lazy person!
For example: the Underground Man once knew a guy who was a connoisseur of Lafitte (a French red wine). That was pretty much the guy's only defining feature, but at least he had a defining feature. So he died a happy (and probably drunk) man.
The point is that the Underground Man has no such vocation. If he could pick something to be, he'd pick…mmm…a glutton (someone who eats huge amounts of food). A glutton who can appreciate "everything sublime and beautiful." The more he thinks about it, the more he realizes that should have been what his life was about: appreciating the sublime and beautiful.
He realizes that such a life would probably have meant a lot of crying. Any time someone did anything beautiful or sublime, he would have wept in awe.
He also would have gotten fat and red in the nose.