Not so much is a great expression! It started out as slang amongst young adults, but I notice it more and more in mainstream newspapers and magazines, for example in this editorial from one of Canada’s major newspapers:
National Post editorial board: Michaelle Jean is doing a great job; Michael Ignatieff not so much
In English, not so much is used the like expression not really. It’s a less forceful or less definitive way of saying no. Saying not so much is also a way of being funny! You use this when you want to dismiss an idea, or say that you didn’t really like something.
NOTE: using not so much this way has nothing to do with the adverb so much.
Afghanistan loves General McChrystal. Eikenberry? Not so much. [Foreign Policy magazine]
China Likes Euro, Yen; Dollar Not So Much [China Investor]
This used to be a Conservative issue. Today, not so much. [Maclean’s magazine]
Elvis Presley was so good-looking in his younger years! In his older years? Not so much!
Don’t worry if you don’t feel comfortable using not so much in conversation. Now you’ll know what it means when you see it/hear it, and the more times you see it/hear it, the more comfortable you’ll become with it!