I think thats a good move for you. I like your lineup more with Andre than without- plus Mendenhall doesn't get many receptions, on top of the fact that he's underproducing. And If you can trade a guy on your bench to upgrade a starting spot, thats generally a good move in my book.
there's no doubt that I would do that trade - you win BIG time! Mendenhall may or may not produce this season, but he isn't worth as much as similar and even lesser RB's in a PPR format and you are getting someone who is much more consistent with the potential to breakout every week. Also, I'd rather start Manningham over D. Moore most weeks (though we'll have to see how he recovers from his concussion), so you aren't really losing much there.
I'd make that deal. Johnson is a beast and I'm personally pretty down on Mendenhall this year. Running backs that go to the Super Bowl tend to underproduce the following year and I think Mendenhall is falling into that category.