So, how much mobile data did Discord use on the phones which ran this view bot in the background for a stupid joke?

None, it was desktop only

TFA mentions it playing on loop in an "in-app toast" do you have a source about it being desktop only?

I never saw it, since I only use the browser version on desktop, and never used/updated the app near april 1st.

This site (ign apparently) is incredibly sketchy without an adblocker on.