It's really a hotel.

The writer thinks the architecture is ugly. The hotel is made of concrete. An "abomination" is something very disgusting. He thinks the hotel building is "a concrete abomination." 

A masquerade is a literally a masked ball, a dance in people meet and dance with each other without knowing who they are. Figuratively, a masquerade means something pretending to be something else.

A hotel means a place that provides rooms for travelers. If this ugly building provides rooms for travelers, it's a hotel. It's just an ugly hotel.

The writer pretends that the word "hotel" means "a beautiful, elegant, and gracious building that provides rooms for travelers." According to the writer, this building is so ugly that it is not "really" a hotel. It is "really" a concrete abomination, wearing a false mask as "a hotel."