This approach is real real common in aggressive pick playing - you can find it in just about any punk / metal band where the bass is belt level or below.
When you have a low slung bass (as Mike Dirnt definitely does) you can’t approach the fret board with classic technique.
You options are: hike up the strap to chest level and look like some kind of square (not acceptable)
Sling it low, and find a way to make it work.
The cool thing about the low slung bass and the fretting hand, if you’re playing with a pick, you can really go nuts with the picking hand. The different grip with the fretting hand frees up the thumb and makes it a very useful muting tool for the lower strings.
That way you can slam away with the pick and not have to worry about accuracy too much and still get out the note you need to get out.
I love playing like this. It’s an adapted technique for a very specific live appearance and attitude - it’s certainly bad technique (and hard on the back).
But it feels real good when you’re standing in front of a wall of speakers and the drummer is playing as hard as they possibly can.
(Chris Novoselic of Nirvana)