You may be shocked to learn how much of the world's "important data" lives in Excel files.
As far as important data served from a relational database is concerned, MySQL most likely beats the hell out of everything in terms of popularity. The bulk of SQLite’s usage has to be various runtime caches, file data, and so on.
we have been living in a psql era for longer than mysql, so it has had enough time to catchup.
If you feel otherwise it's probably that memory bias effect where distant events seem closer and we underestimate the passage of time,
There's an xkcd for that
"The death of mysql(2010) is closer to its birth(1995) than to the present (2025)" will become a true statement next year.
It's obviously going to be hard to determine a number for a statistic like "most popular", and there are always very different interpretations of the available numbers, ending up with vastly different rankings.
I.e. this one keeps Oracle DB at place 1, mysql at second: https://db-engines.com/en/ranking
It links to their method at the top of the statistics. (Oracle has basically a DB monopoly if you believe that ranking, as these are both owned by them)
It's dataset is probably flawed, but I think we can say
"Rumors of mysqls death have been greatly exaggerated"
Easier to copy/replicate, packaged with a GUI, provider with a commercial relationship, programmatic support ( formulas, vba, COM, raw data access)
Would be closer to sqlite than server side though, but it's capacity for replication and sharing makes it more adecuate for sqlite.
Sqlite's lack of GUI gives data a programmer bias, important data uses excel because unless you are in a technocratic environment.