I think a few problems existed for Mars Needs Moms. The biggest being that it looked uninteresting. I treat the 12yr old next door to a movie once a month for walking my dogs and his words were "the Mars movie looks dumb, lets see Rango".
-It looked dumb
-Good competition in Rango
If a movie already looks questionable audiences, it seems, aren't inclined to pay the upcharge and gamble on the flick. Audiences, imo, are starting to wise up to this 3-D gimmick which it has quickly become. Splashing 3-D into the film for a few minutes is not getting value for your $$$$.
That said had it been in straight 2-D it would've only done marginally better. Accounting for the gap in price between the two formats. It's biggest problem was the concept is dumb to it's target audience. I don't care how well the book(s) sold.