If you're looking for a slightly lighter Soviet composer, you might try Andrei Eshpai--he has a number of accessible symphonies, and various orchestral works that, though informed by folk material, are several steps above socialist realism. I recently heard his ballet "A Circle: Apocalypse," a sometimes bizarre concoction of 70's fusion jazz and brass-heavy, Shostakovich-style orchestral bombast. One of his best-known pieces, the Concerto Grosso for Trumpet, Piano, Vibraphone, and Double-Bass, is excellent, and in my opinion is just as good, if not better than, Leonard Bernstein's attempts at classical-jazz fusion. He has also a good saxophone concerto.
I've tried to listen to as much Mossolov as I can find. His Piano Concerto #1 is sometimes staggering, and contains a final, pseudo-jazzy scherzo movement with virtuosic woodwind writing that is just mind-boggling (it's available on a rare Melodiya CD). I've heard a number of his piano sonatas, some songs, a couple of quartets, and miscellaneous piano music--it's all valuable, though I think he was so wrapped up in modernism that he was incapable of writing quiet music. Even his Nocturnes for Piano are noisy. His short opera, "The Hero," was revived in Germany in 1989, and supposedly is brilliant. Unfortunately, as far as I know some of his works are lost (i.e., some of the symphonies), and after the 30s he was "rehabilitated" by the Stalinists (i.e., sent to a concentration camp), and thereafter had to abandon his modernist ways. His Cello Concerto of the late 1930s is dull and hardly recognizable as an authentic work.
I recall reading an excerpt from one of Prokofiev's diaries of the 1920's, where he described judging a piano competition of various up-and-coming Soviet composers. Prokofiev ranked Mossolov at the top, and recommended him even above Shostakovich as the young composer to watch.
Andrew