When you say "Doom", did you mean Poom? Cause the original version of Doom didn't have mouse controls.
Ah, I guess you're right. I still have Doom, but I'd forgotten what options were in the setup program. If that's what it was designed for, it's kinda weird that Doom defaulted to keyboard only, especially given that the input devices are set in a separate program. The controls would have needed to be completely changed to fit too.
I played a bit further, after noticing that the shotgun enemies don't seem to deal any damage if the player is moving. The shotgun seems to be easier to use against the enemies that are introduced after the chaingun.