Or Monzo might even be able to load a virtual card into your Apple / Android wallet for you to use while you’re waiting for the card to arrive 
We are assuming that users names will be on the debit cards here though. If they’re not (to speed up the dispatch process), then cards will be sent out at random (just as they are now) & Monzo probably wouldn’t know who was going to receive which card (unless they use a process like the one Richard suggested) until they register it in the app…