I think everything should be pixelated, not just the characters and some of the backgrounds. To me it looks kinda throw together because some of the graphics have this hand drawn look and others have the pixely look.
I don't see it as a clash of styles really, just that antialisaing doesn't mix well with hand drawn aliased art.
So if you redraw the foreground plant and that Spongebob icon it will all be fine.