apologies for the obvious, as above, capital ‘M’ or lower case ‘m’
I’d hazard a guess that it’d be a capital - all literature has the uppercase ‘m’ apart from the logo.
The company registration is Monzo Bank Limited and the name change document confirms this.
Name change notification - clicky clicky
Capital 
however on the card/branding, its lowercase ‘m’
It seems to be a lower case m when written in that specific form as the logotype, upper case M when written in text.