Is a modern Mustang or a Camaro a sleeper? Nope, so why would a modern Challenger be one. In fact, they are the opposite, they look fast but many are not as fast as we expect, because now we expect all Challengers to run sub 10 sec. like a Demon and when one can't it is a disappointment.
Given how fast some mini vans, SUV's and sedans are now, there are no sleepers.
You want a sleeper get a Tesla 3 and pull the badges off it. In fact, don't line your Challenger up against a Tesla unless you want to get spanked. Watch out for old diesel trucks too.
You know what could be a sleeper, any old car from the 60's and 70's like a Lemans, Tempest, Firebird, Camaro, Mustang, Challenger or Barracuda with a swapped in Modern drive train. Put a Hellcat drive train in an old Challenger or Charger, leave the 318/340 badging on it. Now you have a sleeper.
There was a guy putting a old Firebird body on a Tesla platform, that would do it too.