Lowest I've ever seen was a hunter named Nimrod (you know, the Biblical hunter) in TBC. We were doing Heroic Sethekk as a guild, needed one dps. Picked up this guy. He never topped 300 dps. Only meleed, never had a pet. Asked him some questions and he ran out of ammunition and his pet morale was too low to use his pet because he never fed it. He was decked out in epics too, all welfare though.
Did a HH run with a hunter who did 7k DPS. I had to tell her to send her pet in...
Did Stone Guard with a fire mage who was trying (I think) and he pulled 19k (most of his damage on the stone form add).
WoW is a game of information; the more you know, the better you are.
I don't PvE much but probably me. Back in the dragon soul LFR days I would afk / auto attack in raid finder because I wanted vial of shadows for my rogue. I hate PvE so much, couldn't contribute more than that.
lowest DPS ive seen was 12... some lvl 13 Paladin who helped me killing Anok'suten in Suncrown Village... I kicked him immediately after the mob was down...
at lvl 90 - I think some Rogue with 11k DPS - I didnt care, Im all the DPS the group needs, but not the one it deserves right now.... BECAUSE IM BATMAN!