Is ForeFlight Mobile a 64-bit app?

Yes, version 9 of ForeFlight Mobile is a 64-bit app.  

We expect that ForeFlight Mobile will be fully compatible with iOS 11 once it is released later this summer. However we recommend you wait to install iOS 11 until we have completed compatibility testing with ForeFlight Mobile. Stay tuned to http://blog.foreflight.com for news once iOS 11 is available.