I’m learning an idiom “not to mention” and I saw a sentence in a dictionary as below:
Bananas were scarce, not to mention mangoes.
Is it okay to rewrite a sentence as below:
It’s not easy to find bananas in that country, not to mention mangoes.
Thank you!!!
Yes, that’s OK. It sounds like your issue is more with ‘scarce’ than ‘not to mention’ 
Totally, acceptable. Here’s a similar sentence with a little more context.
“Asian people are everywhere in Taiwan, black people are scarce, not to mention American Indians, which are impossible to find and one of which I’ve never even seen.”