Why does my banana bread have a moist top?
You've partially answered your own question:


When wrapped in foil, the water contained naturally in the ingredients will re-moisturise the banana bread.
a. To reduce:


don't wrap it
leave it in the oven to cool down with the oven slightly open so that most of the moisture can escape


b. to enhance:


make a dome of tin foil above it before putting it in the oven
Wrap it in micro-wave resistant plastic foil the moment it comes out of the oven



P.S. Don't use Saran foil when it comes straight out of the oven.  I've tried and it's not pretty!!!  :-(
P.P.S. For perfect balance, I don't wrap it in tin foil before putting it in the oven but do leave it out for 5 minutes before wrapping it in microwave-resistant plastic foil, but my mum is not your mum, so YMMV.  ;-)
P.P.P.S. Unless you're my brother, in which case your mum is my mum and I want to know: Why didn't you call mum yesterday??? :D :D :D