Just reading through the which result How safe is online banking? - Which?
It seems their main problems with Monzo were probably over passwords (it doesn’t have any and shouldn’t) and the fact it doesn’t log you out of the app every five minutes (please god no).
Just by not having a desktop interface that allows you to make transfers etc it’s already several times more secure than a bank that does, they don’t mention that I don’t think.
Ultimately the breaking the security of your Monzo account requires access to your device or your email, and your PIN number. If one or much more preferably both of those things are secure there’s nothing to worry about.
They mention things like pro security testing tools, but don’t actually say they found any weaknesses anywhere that would allow account access, almost certainly because they didn’t.