I stand corrected. It’s quite clear that there is a machlokos whether marriage is a mitzvah or not.
The rambam States in Ishus “1,2” and in sefer hamitzvas “213” see the magid Mishnah there, clearly that it is a mitzvah.
Similarly the sefer hachinuch “552” as well holds it’s a mitzvah.
Apparently the Smag & Smak as well hold that shita, but I haven’t yet seen them inside.
On the other hand the Ran in the beginning of the second perek of Kiddushin, on the Gemara “Bo yoser mebeshlucho” says the mitzvah is “pru uvru” the achronim are medayik, I saw in Reb Shmuel’s shiurim that there is no mitvah of nissuin or kedushin.
The Rushin Kesubos similarly asks a question why the bracha of the Chupa isn’t a short mitzvah bracha, from this he concludes that there is no mitzvahs kedushin-nesuin.
I didn’t find any mention of a mitzvah neither in tur or Shulchan Oruch.
On the other hand the Revash 398 (see comments by the Mechon Yerushalayim) asks an opposite kasha as that of the Rush. Why is there bechlal a bracha by kedushin-nesuin if there is no mitzvah, anf it’s only dinim. He then argues with rav chasdai, and concludes it’s definitely a mitzvah. He answers the Rush’ kasha by saying that the lengthy bracha is tosfes shevach for this important mitzvah.
Apparently the Shach as well paskens it’s a mitzvah, but I haven’t seen that inside yet either..
The Derech Hamelech (I saw this in the M”Y notes) mentions beginning of hilchos ishus, that the Menchas Chinuc when he was mesader kedushin, would tell the chasan to have kavanahs to be yotze mitzvahs esseh of the those shitas who hold it’s a mitzvah.