It was reinforced heavily in the early days.
From workplace diversity to critiques of common banking practices to helping refugees open accounts to paying attention to the mental wellbeing of their staff (and their customers) to helping gambling self-exclusion to having readable Ts&Cs to their tone of voice to attempting to have a fairer overdraft pricing structure.
Transparency was also heavily emphasised for a long time, which can be surprising looking at the company now.
And of course, their June 2016 post titled What Makes a Bank Ethical?:
We started Mondo because we believe there should be an alternative to the banking of the past. We are focussing on solving customers’ problems, rather than selling financial products, to create what we hope will be the best current account in the world.
Unfortunetly the Monzo Ethics page linked from it is laughably lacking in information.