It was deliberate. As were the instructions Hitch gave Ron. He wanted lively music like a travel film about London, "to relax the audience and to catch them off guard with the shock of the body in the water...!"
When Mancini was dumped it must have been a pretty big deal in music circles, him being probably the hottest film composer in the world at the time (ok John Barry too).
The situation appears to mirror the Bernard Herrmann-Richard Rodney Bennett "debate" about Murder On The Orient Express, where Herrmann thought the music should take things DEAD seriously, but Bennett and Lumet were savvy enough to realise otherwise.
Hitchcock made a very VERY meta black comedy, and Goodwin was there to, as we say in the UK, "take the piss" a little. As someone else said, who knows if Ron was entirely in on the gag. Very good score in any event.
Very good score in any event. 
