As far as comedy goes in the realm of Welles...well, humor is a very personalized thing for everybody. Without going into the scientific hubris, there are some people who think that Adam Sandler screaming like an incessant man-child is absolutely hilarious, while there are others who prefer humorous word-play, or clever sight gags, etc. I have to say that, while I don't think Welles was the greatest screen comedian ever, I don't think he was completely terrible. Granted, there were actors in his films who were certainly funnier than he was (the scene in Ambersons when Fanny starts shrieking at George right after the ball is pretty funny), and I think that was the key to whether his comedy worked or not.
I guess the argument regarding timing is worthwhile as well, though that has plagued a lot of directors. For example, a lot of people think that the end of the scene where James Gregory asks Angela Landsbury to give him "one simple number that would be easy for me to remember" in The Manchurian Candidate is pretty hilarious, but by the time that he delivered the punchline, I had already chuckled a bit (I mean, the bottle of Heinz was way too obvious) and moved on with the picture (myself personally, I found more humor in the incredibly bizarre conversation on the train..."Are you Arabic?"). Clever? Yes, but I also think that, say, a Biff Rose record from the late-60s is pretty clever as well, and I'm more likely to laugh at that.
That having been said, I think that the major part of the problem with "Swinging London" is that Welles was trying his hand at what is particularly (if you'll pardon the expression) British humor with those segments. Out of all of them, I thought that the tailor and Plumfield Manor segments were amusing, the Churchill bit was okay (but not nearly as amusing), and the "Carnaby Street" was over-done (though Orson's 'singing' is pretty funny). Overall, though, it certainly didn't leave me in stitches.