Charlie Kelly is easily the dumbest character on It's Always Sunny in Philadelphia. While he may not be the most psychotic, and while he may be the most "friendly" and "outgoing" of the group—and that is said very loosely—he is easily the dumbest.
With that said, Charlie has proved himself a bit of an idiot savant throughout the years. He does have a few talents; he has a better social game than the rest of the group—again, saying that very loosely—and he has proven himself a good liar and manipulator on numerous occasions. These are ten times that Charlie Kelly was actually intelligent.
He Is Musically Gifted
Maybe in another life Charlie had a successful career as a musician, as it's clear that he is very gifted. He has written numerous songs throughout the seasons including "I Like Paddy's Pub," which was legitimately good, and he even wrote an entire musical himself. Granted, the musical didn't make much sense and contained many unintentionally inappropriate sections, but it still proves his drive and ambition. He can also play the piano, which is certainly not an easy thing to do.
He Has Perfect Pitch
Speaking of the whole "musically gifted" thing, Charlie has perfect pitch. This is when someone can recognize a musical note without a reference tone, and it is an exceedingly rare gift to have. Charlie demonstrates his perfect pitch when the carbon monoxide detector goes off. He recognizes the beep as a G-sharp note, and Frank confirms this when he plays the same note on a recorder. Charlie really missed his calling in life.
He Singlehandedly Keeps The Bar Running
If it weren't for Charlie, then Paddy's Pub would be in absolute shambles. This is because Dennis, Dee, Frank, and Mac don't do a single thing to keep it running—at least, not on a surface level. Charlie does all the janitorial work, which includes killing rats and cleaning toilets, he changes out the kegs, disposes of the trash, and ensures that the bar passes health inspections. Say what you will about the man, but he's a very hard and very smart worker.
The Charlie Work Scheme
Speaking of passing health inspections... that's not exactly on the up-and-up.
But it does show off Charlie's insane brilliance. During the surprise health inspection, Charlie moves the people around like chess pieces with precise timing and precision. He successfully manipulates both the delivery man and the health inspector and ensures that the bar passes the inspection. Charlie is a certifiable chess master.
His Waitress Scheme
Charlie's scheme to sleep with the waitress is actually pretty brilliant. He manipulates Mac into sleeping with Mrs. Reynolds, then manipulates Dennis into sleeping with Mac's mother out of revenge. He also allies himself with Dee and uses her to convince Dennis that he should sleep with Charlie's mom. He attempts this, as well, which destroys the waitress. Dee then tells the waitress to sleep with Charlie to pay back Dennis. Of course, she doesn't end up doing that, but, hey, it was a good plan!
His Other Waitress Scheme
Charlie really fires up the old brain box when it comes to the Waitress. Charlie befriends a rich heiress named Ruby Taft, and the two seem to form a genuine connection throughout the episode. However, we later learn that Charlie was only using Ruby to make the waitress jealous. It's one of the rare times that Charlie shows willing deception to get what he wants, and it proves that he has a pretty firm grasp on human psychology.
He's A Good Sewer
No, not a good "sewer." A good sewer, as in a person who likes sewing. In the episode The Aluminum Monster vs. Fatty Magoo, it is revealed that Charlie often sews his clothing back together, allowing him to maintain their quality throughout the years.
Later, when Mac and Frank open a sweatshop in the basement, Charlie proves incredibly adept at sewing and seems to be the fastest and most efficient member of the team.
He's Great At Dominoes
For an example of Charlie's skills and intelligence, look no further than the very first episode of It's Always Sunny in Philadelphia. After Mac unsuccessfully tries to break the ice with some prospective black friends at a college campus, Charlie fits right in and receives praise for his dominating performance in dominoes. It proves that Charlie has the mental forethought to plan, scheme, and outsmart. He just...doesn't use those qualities very often.
Fooling Dennis And Dee
Continuing with Charlie's surprisingly adept skills, it's clear that he is an accomplished actor. The entire episode The Gang Broke Dee revolves around an intricate ploy to fool Dee and make her feel depressed. The ploy is concocted and carried out by Mac, Frank, and Charlie. Not only do they successfully fool Dee, but they actually manage to trick Dennis, as well—he wasn't in on the joke. Does this mean that Charlie has the capability to outsmart Dennis?
Dominating In TechPocalypse
Charlie Rules the World is filled with brilliant Charlie moments. At first, he correctly deduces the meaning behind Dee's username (ZingingCutie23—she's pretending to be 23) and correctly claims that she is using the game to feel superior to the gang. But, when he tries the game himself, he absolutely dominates by forming alliances, making deals, and betraying people. It once again proves his manipulative abilities and once again proves that Charlie can be a chess master when he wants to.