Likely the second most widely deployed software ever. https://www.sqlite.org/mostdeployed.html
The best tested piece of software that I know of. https://www.sqlite.org/testing.html
People call it a toy database. Some toy.
In the case of databases, I can well imagine Oracle or even Postgres people thinking that SQLite must be a toy because otherwise all the faff they do to set up and admin (and pay obscene amounts for) the databases is actually pointless.
Stands to reason. It is something you want to play with, not a figurine to leave on the shelf to forget about like some of those other database solutions.
(Taken from https://www.sqlite.org/whentouse.html)
A toy that can serve the vast majority of DB use cases. I get it, you can't build a massive-scale project with SQLite, but that doesn't exactly make it a toy DB...