"Mountain lions" aren't restricted to the mountains by any means (panthers, pumas, cougars, mountain lions, and catamounts - yes, even catamounts! - are all one species,
Felis or sometimes
Puma concolor). They're quite happy in a wide range of environments, mountains, plains, forests, swamps, etc. Most sightings by random Joe Schmoes aren't
actually pumas for sure though, mostly large house cats. It often reminds me of the guy in my condo complex who was frantically taking pictures of a buzzard (turkey vulture), convinced it was a california condor.
http://www.easterncougarnet.org/prairiestates.html has a map of plains sightings.