I had never really thought about it before, but I can definitely hear how R2's speech in Jedi is more comic. This is something that was greatly exaggerated in the prequels (especially in Sith, when R2 just wouldn't stop screaming during the crash landing).
It's really weird because R2 isn't a normal robot by any means. During the Dagobah scenes in Empire, R2 sometimes comes across like more of a pet than an ordinary robot (especially when it's peeking in the windows of Yoda's hut). But I must admit that it is a little strange that R2 displays much more fear at Jabba's, when it knew that Luke wasn't really donating them and that Luke had a plan, than on Dagobah, when R2 really had no idea what to expect.