I don't know. Will the whole Spongebob thing last long enough to justify spending the money on printing sideart and control panel art? Even if it does, will your little girl still be into it in a couple of years? Those are questions you need to answer. I still label "Spongebob" as fad. It may pull a "Simpsons" (be a 'fad', then dip into 'cult status' and then up to 'pop culture') but that doesn't happen all that often.
I've always favored 'simple' over 'complex' when it comes to artwork. Little arrows around the joysticks, maybe some text (like Player 1 Start, etc.) and a simple background. There are some stunning examples of very detailed art, but then I look at the picture and I have to stare at it for 15 minutes to find the joysticks amid the busy colors. That's just me, though. Opinions may and will vary.
Anyway, anything you do is ultimately your choice and you can always change what you don't like. Good luck.