We were there spring 2018 and observed Happily Ever After (HEA) from three spots, Main Street USA, the Contemporary California Grill Observation Platform (South, closer to Wilderness Lodge end), and the Contemporary Concourse (4th floor) deck. To us, in our opinion, while all three work, the optimum is obviously in park itself. 2nd would be California Grill (all fireworks and music, overall entire atmosphere and surroundings) with less visual detail on the projections. 3rd would be the Concourse (nearly all the fireworks and all the music, virtually no visual detail on the projections).
So, to answer the original question, yes, HEA is observable from the Contemporary Resort, but, in our opinion, not to the same extent as possible while in the Magic Kingdom.
As many have said, access to the roof deck is reserved for California Grill diners. You can however, just walk out the north end of the Grand Canyon Concourse level (4th floor) and watch them from there. The area underneath the monorail beam can get congested so what I've done many times is just go over to the evacuation stairs on the Bay Lake side and walk up a couple of levels. You'll be above the monorail beam and have a great view of the whole show and still hear the soundtrack. It's certainly not the same show as standing on MSUSA or the hub but it's a whole lot less congestion during and after.