As long as some here are already rolling their eyes at interpreting Fett's characterization via context (I see it the same way you do), I'll take it to the next level by trying to justify those ROTJ moments.
In ESB, Fett learned that Luke Skywalker was the guy who Vader (and thereby the Empire) was obsessed enough with to go to great lengths to capture him alive. In ROTJ, Fett suddenly has Luke walk right in to where he was hanging out. Taking Luke alive and bringing him to Vader would have to be the greatest bounty of his career. I can see that being on his mind during the ROTJ events.
When Fett lands on the skiff, it doesn't look like he's even aiming at Luke (seriously, watch the aim placement). He might've been trying to clear out the others behind him and off to the side. And when Luke is distracted and not even looking, Fett uses a lasso rather than the wrist rockets. That lasso tells me he's trying to wrangle him rather than kill him the whole time.
Then come those very errant wrist rocket shots. This is tougher. But maybe he was simply trying to put distance between Luke and Jabba's goons. Maybe he was aiming for the lightsaber (a tough moving target) to merely disarm his potential quarry. (I know, I know )
Or forget all that and just assume Fett was totally wasted on whatever Jabba had coming out of those hookah pipes. I'm good with either explanation. Better than accepting complete and total incompetence, right?
It would have been nice if when George was updating ROTJ he added some sparks to Fett's wrist rocket launcher to indicate that it was damaged and that maybe the targeting was off or something. But no add some silly new cables around Han Solo's feet instead, lol.