This doesn't necessarily mean that the data was already on the 3DS card, because the download of the 20 new Mutant Mudd levels finished within a minute too. Each DLC pack for NSMB2 is just three levels, so it's barely any data that needs to be transfered. All the DLC packs' game code and assets are already on the 3DS card, so Nintendo is essentially only transfering level editor data.
You could be right. I think the Mutant Mudds levels took longer to download, though. Then again, those Nes Ambassador titles downloaded quickly, as well. I'll look at my 3DS and see how big the download files are--but that won't really prove anything, I guess.