Is Vonnegut an optimist?
I congratulate you on doing some thinking. Whether you're right or wrong about Vonnegut you are to be commended. I do suggest, however, that you're misinterpreting Vonnegut. I've heard Vonnegut called many things, but an optimist is not one of them.
In general, as well as in the quote you cite, Vonnegut uses detached irony and understatement. You should not interpret this as optimism.
When he says that he would rather laugh than cry because it takes less cleaning up, the emphasis is on the frustration and exhaustion. The humor creates understatement. Rather than attempting to say and emphasize how terrible the frustration and exhaustion is, Vonnegut uses humor and understatement to let the reader create the meaning of the frustration and exhaustion. In other words, the reader concludes how serious the frustration and exhaustion are. This way, Vonnegut isn't trying to convince the reader of anything--the reader does it alone. At least that's the appearance.
Even if the above explanation is confusing and even if I'm incorrect about the specifics, though, your interpretation of Vonnegut comes down to your viewing Vonnegut's suggestion of laughter as optimistic. That is a mistake. Vonnegut laughs because of despair. Vonnegut laughs because there is nothing else he can do. Vonnegut laughs because he has no control over what happens. Laughing is simply better than crying, but it's not optimistic. The emphasis is on Vonnegut's targets, but pessimism is not normally one of Vonnegut's targets. People who take themselves too seriously, blind patriotism, short-sighted humans, irresponsible science, cruelty, war, etc., are usual targets of Vonnegut.
That said, Vonnegut is not a nihilist. Nihilism is an occasional target in Vonnegut. Humans and human events and human sufferings do matter to Vonnegut. And targeting human shortcomings may suggest a belief in the capacity for humans to change. This, in an offbeat way, may indicate that Vonnegut is a bit optimistic.
Finally, and I'm being thorough because it is obvious that you've done quite a bit of thinking, be careful of dichotomies. When you say that most people are either optimistic or pessimistic, you are dividing people into two, definite parts. Human existence isn't usually that simple. Most people are not one or the other of anything. People are too complex for that. We are mixtures of optimism and pessimism. We are not completely good or completely bad, completely pure or completely impure, etc. We are mixtures. Dichotomies are usually too simplistic and too absolute. They are usually inaccurate.