Strickly speaking, there are no "mods" in Xonotic. However, sometimes the term "mods" is loosely used to refer to variations of the officially released game.
For example, there used to be an "SMB mod pack" with experimental features that were not in the official game. The developer of SMB Mod Pack is Mario, who is in the Xonotic core development team.
Another "Xonotic mod", ChaosEsque-Anthology, based its code on the official Xonotic, but this project has no official connection to Xonotic.