The sentence from chapter one of
Slaughterhouse-Five that states, "We were Mutt and Jeff in the war" is accompanied by my enthusiastic annotation of "Yes, Mutt and Jeff!!! I kind of know what that alludes to." (The excitement of feeling smart lead to all those exclamation points.) But since I only "kind of" knew what it was referring to, I decided to look it up. I've only heard of the term once before, when a longtime friend of mine's mother decided to start calling the two of us Mutt and Jeff, since we vary in height by about five inches. We were both really confused, and she explained to us that
Mutt and Jeff was an old comic strip with one character that's ridiculously tall and lanky, and the other short and squat. The names stuck.
Wikipedia informed me that the American comic strip written by Bud Fisher was extremely popular in the early 1900s. It was something of a slapstick comedy that followed the adventures of two friends. Interestingly enough, Mutt and Jeff were also codenames for two WWII spies who worked for the UK and lead Germans to think that the 1944 D-Day landings would be in Calais, not Normandy. It seems that Vonnegut would be aware of this connection, but I'm not sure if it was intentional.
The comic strip version. But Britain did have an airborne division in the D-Day invasion...