Can a fully honed Mandalorian beat a random Jedi? You bet.
Yes they can beat "a jedi". However, If you throw a fully honed Mandalorian against specific force users such as Darth Vader, Caedus Sidious, Bane, Malgus, Luke Skywalker (when jedi master,) Mara Jade Skywalker, Jaina Solo, Kyle Katarn, Yoda etc.
I wouldn't want to be the Mandalorian. However, some random jedi? Sure.