This is been partly discussed here:
I think this may be something for the future, but not a focus for the next year or two, Monzo needs to be profitable first, then can look at things like this.
That said, I’m sure they could look to do little steps towards this, and if they were really all about being ‘greener’ then they’d make it a higher priority, but they’ve never said they are focussed on things like this so who knows.