Yes, yes, yes.
This is one of the most basic features to be expected in just about every game and Splatoon 2 does not have it, and to me that's just plain inexcusable. I can't imagine it'd be hard to program in, so why is it not there?
It doesn't matter how good the OST is, once you've played and practiced it for hundreds of hours you'll get burned out on the in-game OST eventually. You could listen to your own stuff, but Splatoon 2 won't let you. Either you gotta mute the entire game or have to suffer through Ink Theory for the umpteenth time, or have your own music blend seamlessly with that of the game itself.
Even outside of muting the ost, there are people who just like to have the OST a little quieter just so they can hear sound cues a little better. Again, Splatoon 2 won't let you.