Playing the level backwards feels like a gimmick they would add to a rudderless game. I hope they nail the level design.

But Wario games have been doing that a bit with a backwards dash at the end of the level. WarioLand 4 and Shake It. What's wrong with that idea?

I imagine how it's implemented.  Wario Land 4 and Shake usually introduce new area's on the way back that players have to go through or put in some pretty substantial new challenge in the old area.  Plus they have a timer which forces the player to have to go faster which changes up the gameplay as well.

As long as the journey back to the beginning changes things up so it doesn't just feel like a pointless backtrack to the start then I don't see the big deal.  If it's mostly the same level though with just a few new items and enemies then I could see why some people would find that not very fun.

They need to go all in on the backwards theme. Backwards music! All enemies Yoshi ate get spit out on the way back! Backwards controls! Fall up pits! Enemies jump on Yoshi to be defeated!