Spoilers for Deadpool 2 below!
After ending up locked up in the "Ice Box," an isolated prison for mutant criminals, Wade Wilson/Deadpool (Ryan Reynolds) is fitted with an inhibitor collar that negates his mutant powers. For Deadpool, that's a problem. Without his healing factor, his aggressive cancer can ravage his body unchecked which causes his body to grow weaker and weaker. A weakened Deadpool is of little use in helping Russell Collins/Firefist (Julian Dennison) escape. This status makes the Merc claim he's essentially useless, going so far to say that someone should just give him a bow and arrow as he's so useless he might as well be Hawkeye.
The stinging reference takes advantage of the joke that, as the only non-super powered Avenger, Clint Barton/Hawkeye (played by Jeremy Renner in the Marvel Cinematic Universe) is less useful than the rest of the team. After all, what does he really have other than his archery skills? While one can argue that those archery skills make Hawkeye a powerful ally and very useful, the joke ends up being truly funny thanks to Hawkeye's role -- or lack thereof -- in Avengers: Infinity War.
You see, Hawkeye really was "useless" in Infinity War. On house arrest thanks to a deal made after the events of Captain America: Civil War, Hawkeye was off the grid while the rest of the heroes faced off against Thanos. Presumably, Hawkeye was with his family on the farm, unaware of the devastation coming when the Mad Titan snapped his fingers.
Given that the "Snappening" radically reduced the living population of the universe by half, odds are pretty good that Hawkeye knows something's going on now. Based on set photos from the upcoming Avengers 4, Hawkeye appears to have not only survived but may be poised to play an important role in what comes after Infinity War. That will certainly make Deadpool's joke about the archer being useless well, useless, but for now, it's a pretty funny "dis" and it wasn't the only one in the film. After all, when Deadpool and Cable (Josh Brolin) team up later in the film, Deadpool breaks the fourth wall by referring the Cable as Thanos -- Brolin's character in Infinity War.
Deadpool 2 is now in theaters. The X-Men movie universe will continue with X-Men: Dark Phoenix on February 14, 2019 and The New Mutants on August 2, 2019.