Deadpool, the Merc with a Mouth played by Ryan Reynolds, is still trying to find his footing in Disney's MCU, with his famous R-rating facing an uncertain future. Many contend that the foul-mouthed mercenary has always been R-rated since his inception, but the comics tell a very different story.
Deadpool first appeared in The New Mutants #98 in 1991. At the time, all of Marvel Comics were approved by the Comic Code Authority. By then, the Code had been mostly watered-down, a far cry from its 1954 counterpart. Still, the Code did approve Marvel's titles for a general comic book audience. However, in a decade, Marvel would cease submitting to the Comic Code Authority, along many other publishers, in favor of a traditional rating system similar to the ones established by the MPAA and ESRB for movies and video games, respectively. Even then, Deadpool maintained a comfortable teen rating. Case in point, Deadpool's current solo title, written by Kelly Thompson, is written for ages "12+ Only".
There are a few exceptions. Deadpool Killustrated, which features the mercenary targeting literary characters, has a mature 17 and older rating. So does Deadpool MAX, a series of comic books from Marvel's mature-reader imprint of the same name. However, these are the exception, not the rule. Even titles with wildly violent premises, such as the aptly-named Deadpool Kills the Marvel Universe still bear the traditional "12+ Only" rating.
There are a couple of caveats to just what passes for PG-13 in a comic book though, by virtue of comic book's visual medium. For instance, many characters still swear, or at least, are implied to swear, with their dialogue represented by symbols such as "@#$%" or are blanked-out altogether. Deadpool is certainly no exception to this, though different writers have depicted his humor and dialogue in different ways over the years, with some leaning further into the merc with a foul mouth than others.
However, this doesn't guarantee smooth sailings for Deadpool's movie counterpart. The success of the Deadpool franchise has been largely defined by its no-holds-barred approach, something that has convinced movie executives that R-rated superhero flick could be profitable. If and how Deadpool will fare in the PG-13 constraints of the Marvel Cinematic Universe remains to be seen, but his comic book history indicates that the source material for toned-down Deadpool exists. Still, Wade Wilson's fourth-wall-breaking-antics all but ensure that whatever rating Deadpool has to work under, the results will be nothing if not fun to watch.