So I had a bit of a stab at some numbers on the carbon footprint of a metal card vs a plastic one; here
The tl;dr of it is, with some very fuzzy assumptions, a metal card is about 3x more carbon than a plastic one, clocking in at 60g CO2 and 20g CO2 respectively.
If you compare that to, say, driving, a fairly modern car (volvo V40) emits about 140g CO2 in 1 mile of driving… (source: here).
So in the grand scheme… As a consumer - as you say - there are 1,000 other things above and beyond a metal card in terms of impact.
And as for Monzo as an organisation - I would be more concerned with making sure their operation is carbon friendly; things like servers, heating, electricity would have a big impact, and more than likely all of their staff commuting to their offices costs more in carbon than the total of their card production.
And in fact, per this article, sending a new card in the mail has a carbon impact of about 25g… So theoretically if a metal card can last 3x longer you’re saving an additional 60g+ of carbon.
Anyway.