I remember the gamespy stats charts having Quake 3 around 3k players which was indeed huge in it's day but absolute chump change now. Quake 3 was always eclipsed by Half Life on the stats though because of CS. Once CS came out it was just a downward trend for all Quake games.
It's honestly insane how big gaming is now compared to then, so it's hard to call something really popular when the numbers were so small, but in comparison to other games it was quite big yes.
I have the time but it doesn't help you through never ending why
Brian Molko, Battle for the sun
the idea i want to express and communicate it may disappear in a couple of seconds. Your question of how popular lost its importance just within a minute
True, QC splitted the community and failed to deliver, the duel format was bad, the meta champions adds even more depth, the netcode was bad. I was hyped and played for some time, but it still feels so floaty compared to Quakelive, which is a solid game.