In all honesty, ROTJ Luke is supposed to be an exceptional duelist. The sheer stupidity of his fighting abilities is simply that the 70s and 80s hardly had anything close to the choreography and stunt work that we have today.
I recall Luke doing only a single backflip in ROTJ. How many has everyone else done in the PT? Dozens. Dozens.
Point moot.
ROTJ Luke is supposed to be an outstanding duelist for Jedi standards. Am I saying he's on Count Dooku's or Yoda's level at this time? Nope.
I'm saying he is supposed to be portrayed as a highly skilled duelist, especially considering he's not had much training and the choreography sucked.