I did try going online, but all servers I could find were empty.

Here is what I did I went to Party Mode and then Internet play, and all of the servers were either empty or had bots instead of actual players.

I do remember when I was a kid/teenager I used to go online and played a lot of battle modes with other players here, obviously there aren't as many who do that anymore.

People usually play Jazz 2 during evening and night time (Romanian time. I can't tell what time would be for you). Servers are usually empty on working days (except friday). Try joining on a saturday night, I can almost guatantee that you will find at least a few players.

JJ2 multiplayer is becoming inactive now a days.

There are way too many servers and way too few players at this time. I remember that in 2009, the only dedicated servers were "The Server", "Zeal Alpha" and "Zeal Duels" after midnight (CET). People were hosting their own servers at daytime. Now that barely happens anymore.

The clan ladder seems to be inactive as well. People just play tests or just fool around in the battle servers.

Try playing during the evening (Around CET or GMT) to find most activity, especially in weekends. The clan ladder indeed dropped in activity this season, it's just players losing interest and gaining interest over periods of time for all I know. The current limited clan scene made at least CDF less interested in playing ladders and CW's, JJ2 is more casual play for us lately.