Export CSV of an smart group's contacts via REST API?
You can't create a csv directly from an API call. But it would be a very simple task to make an API call and then format the results as a csv. One simple option might be to use Google Apps Script to call the API then put the results into a spreadsheet, if you don't want to run this as a standalone script. Or any scripting language you might want to use will have plenty of Google results for creating a csv from an array, which is what the API will return.