Hotel reviews in Apple's Maps app aren't limited to Yelp any more, at least for some locations. TripAdvisor and Booking.com reviews are available for some hotels, although the majority of locations are still showing content from Yelp.
Maps hotel reviews now include Booking.com, TripAdvisor
Prior to TripAdvisor and Booking.com showing up, reviews for businesses and restaurants in Maps was exclusively Yelp's territory.
Our checks for several cities shows TripAdvisor and Bookings.com reviews often in cities outside the United States, but the cities we checked in the States were all still limited to Yelp. At least for now, it seems alternative hotel reviews are few and far between in the US, and may be limited to certain cities elsewhere in the world.