"was not crowded" is correct and usable in written English.
It can be used when one is describing a place that lacks people. For example, "The beach was not crowded due to the unpleasant weather conditions."
The place was not crowded.
The driver who took the turnpike said it was not crowded, but it was not empty, either.
It was not crowded on my visits, but it deserves more attention, for this is a very good place.
The next time we went it was a weeknight, it was not crowded, and the experience could not have been more different.
The beach was not crowded, as they expected, but this being the Hamptons and all, they recognized Steven Spielberg enjoying a sandy constitutional.
The famed Boardwalk was not crowded, there was no wait for tables at popular restaurants, tickets were still available days before a Black Eyed Peas concert, and rows of slot machines sat unused, blinking away at no one.
"It wasn't crowded at all," Veras said.
But it wasn't crowded the other night, a Wednesday.
(It wasn't crowded, though, according to the Times).
"I wasn't crowded in any way," Judge Wilkinson said.
I got there at opening time and for twenty minutes or so it wasn't crowded.
