partly because his character is understandably constantly acting nervous and befuddled, which is Stewart's standard shtick. I just generally find him to be incredibly hammy. Haven't seen "It's A Wonderful Life" or "Rope" though.
"Vertigo" is impressive from a technical standpoint but I expect that from Hitchcock. Aside from Stewart's cornball performance, my main issue is the gimmicky resolution, which borders on soap opera-level ridiculousness.