Kilgore Trout is a fictional character. You could ask Kurt Vonnegut, who invented him, but he's dead. You can certainly assume that it's meant to be humorous.

He meant to entertain the reader with a clever turn of phrase. Something good authors are apt to do.

For one, it's a clever, humorous phrase, since the conventional wisdom is that the universe IS the biggest place, so there would be no reason for "perhaps".

Second, it also entertains some doubt: perhaps the universe IS NOT the biggest place. Maybe the conventional wisdom is wrong. Maybe other places exists, from the mythical and spiritual domain (like paradise and hell and Valhala etc), to the scientific domain (multiverses, many worlds interpretation of QM, etc).

Interesting thought. I wonder, though, what makes those places separate from the universe? The universe including those other places, metaphysical and scientific, would be even bigger and more mind-boggling.

Well, you have to define the universe at some point, and the most common definition on people's minds it's "this thing which our galaxy is in". I would presume that common perceptions don't include such notions as "separate reality split-universes" and sets of multi-verses, and what have you.