Are the radar images within ForeFlight Mobile base or composite reflectivity?

The radar imagery shown on the Maps page is composite reflectivity. This is true regardless of whether the source is ADS-B, XM, or via the internet.

All of the radar images on the Imagery page are taken from the lowest elevation level, and are therefore base reflectivity.

To learn more about composite reflectivity, the NWS JetStream Online School for Weather: Composite Reflectivity article is very helpful.

Further, the NWS JetStream Online School for Weather: Base Reflectivity article is a good read to learn more about base reflectivity.