I would like to see this in the app also.
Having been to South Africa twice this year already, I just base my transactions off 17 ZAR to the £1 but this is a rough estimation. I am fine with the end result being slightly off but it would be good to know I can look it up. For example the ZAR exchange rate changed 7% while I was in SA last time and I had no idea what that meant for me when I was spending.
I think as an idea when you get the welcome to SA notification in monzo if you could click that and see the exchange rate at the exact moment you click it and then be able to go back and click it again and see it current price at that moment that would be really good. Might not be so helpful for people in the UK paying in foreign exchange but would be good for me when traveling