There are at least 50 songs in the Genesis oeuvre worse than that cute little ditty, which is in many ways one of the more experimental tunes on the album, together with Keep It Dark..Admittedly, most of the really bad tunes came after Abacab, so it was a bit of a low point at the time. even if they had turned out some even worse stinkers before that, like More Fool Me, Your Special Way and Alone Tonight. Afterwards there were terrible moments aplenty from Illegal Alien and Anything She Does all the way to most of We Can't Dance and every single note on Calling All Stations.
No, Who Dunnit isn't really that bad at all.
For me, the factor that makes Who Dunnit (you're right, it is 2 words...

) more annoying than any other bad song they've done was the REPITITION. Had it varied a bit, or somehow had some other chords or something, it might have been made more bearable for me. As in the case of "Submarine," it too was repititious, but I always found it kinda soothing, so I didn't mind. I've figured out that, for myself, there's a good kind of repitition and a bad kind. Who Dunnit was the living incarnation of the BAD kind for me.
So there ya go.
(Seriously - you feel that "The Dividing Line" and/or "Calling All Stations" the title track are arguably as bad as Who Dunnit?

I always liked those songs quite a bit...)