As others have said deadpool is called merc with the mouth because of his talkative nature and often has a tendency to break the fourth wall . (for those who don't know). "Breaking the fourth wall" means that speaking directly or acknowledging or doing something to the audience. read more
Deadpool is a funny guy who doesn't like to stop talking as well as he regularly breaks the fourth wall. Fourth wall is a scenario where the Deadpool directly addresses the audience. And we know the disastrous mercenary in him, so the term merc with a mouth. read more
This incarnation of Deadpool, frequently referred to as Headpool, entered the mainstream Marvel continuity when he is encountered and captured by the original Deadpool in Deadpool: Merc with a Mouth. Along with several other alternate versions of Deadpool, Headpool went on to appear in Deadpool Corps with a propeller beanie mounted to his head, allowing him flight. read more
Ryan Reynolds’s Deadpool is called the ‘Merc with a Mouth’ for very good reason. In the comics, Wade Wilson (a wink and a nod to DC Comics’ Slade Wilson AKA Deathstroke) joins the infamous Weapon X program – the same organization that fused adamantium into Wolverine’s bones. read more