While I imagine many hardcore comic book fans are going to see Phineas and Ferb: Mission Marvel and roll their eyes, the target market it's aimed at will smile and take it all in gleefully. If Marvel purists couldn't see this marriage of worlds coming a mile away, they were obviously hiding their heads in the sand. If you like it, then watch it. If you don't, just close your eyes to it and move on. That's the best advice I can give.