Sentences are not prescribed. The magistrate(s) will have taken into account the nature and seriousness of the offence, the amounts involved, the defendant’s guilty plea and apology, his explanation for committing the crime, his mental health, his previous (if any) involvement in the justice system. The £20 is not ‘damages’ awarded to the victim.
It’s not a ‘good deal’ – the man still has a criminal conviction. As someone who actually understands how the justice system works, that’s sounds fair and about right to me.
Please also remember that, although there will be some effect to the cardholder (possibly even psychological), the ‘victim’ in this case is Monzo. The cardholder will be covered by the bank and would not have lost any money.