It's a play on several tropes, namely No Time To Explain, Oh Crap!, and Don't Ask, Just Run.
Rather than get into the details of why it's so terrible, Rick, who's the only character present that's safe to assume would know why the planet is so terrible, simply hurries everyone back onto the ship to go to the third and final planet.
What makes the scene funnier: knowing the reason everything is corn shaped is so bad, or simply having Rick panic (Oh Crap!) and forcing his family to leave the planet (Don't Ask, Just Run) for seemingly no reason (No Time To Explain)? Do you really want to know why everything being in cob form is bad, or is it more fun to try and come up with your own explanation? Oftentimes the best monsters are the ones we never see and are left to imagine for ourselves.
Not everything needs to be explained. Doing so frequently can bog down the story in pointless details, make something that's supposed to be a big deal suddenly not a big deal, or maybe the writers couldn't really think of a good reason why but found a way to use an idea they came up with anyway.