NRPN has max value of 16383, not 127. Try to set it to that.
In the generic remote XML, add 16 to the flag. Instead of 5, make it 21.
Try to disable the Relative flag, since Relative needs 1 and 127 for down and up (or 1 and 16383 when using nrpn).
I’m not sure how Cubase handles incr/decr, but definitely not using Relative mode.
If nothing works, can the MPD send these kinds of messages instead of incr/decr?
Another thing to check is whether the MPD has memory. Try to set the midi output to it and add the “Transmit” flag.