The Monzo app doesn’t have a PIN lock to open it 
That’s something we’re considering for the future though 
(optionally) 
It’s worth noting that the app is essentially ‘read only’ when you open it You still need to enter your card’s PIN to send money to friends / setup bank transfers, view statements, etc. 
In the future, we’d love to support Pixel 4’s shiny new face unlock though 