Friends first aired in 1994, and by its last episode in 2004, Joey had gotten a lot dumber. The show ran for 10 seasons, making it one of the most successful and well-loved American sitcoms of all time. Over the course of its 10 years, the main group of characters — Chandler, Ross, Monica, Phoebe, Rachel, and Joey — grew as the series progressed. Each of the Friends gang's change from season 1 to season 10 is palpable, but it's Joey's devolving intelligence that stands out the most.
In the pilot episode of Friends, Joey Tribbiani (Matt LeBlanc) is immediately characterized as a stereotypical womanizer and slightly naive. This is expanded upon in a later season when the Friends recount how they first met each other. Upon first meeting Monica, Joey took her offer to have lemonade in her apartment as some sort of sexual advancement, misreading the situation completely. He starts the show off by being fairly simple-minded but becomes increasingly feebleminded with a childlike understanding of even the most basic concepts. Examples such as the reference to his ''Joey's apple'' and the infamous scene of Phoebe trying to teach him French show just how much dumber he got.
The reason for this devolution of his intelligence is likely because initially writers were finding their footing with each character and playing it relatively safe for the Friends pilot and the rest of the first season. Once they had the reaction of the audience to play off of, they seem to have played up the parts of the characters that people liked the most and which parts drew the most laughter, leading to most of the Friends group becoming parodies of their original selves. Ross goes full-blown geek, Monica becomes more cleaning-obsessed, and Joey gets a lot dumber.
Joey fits the friend group stereotype of "the dumb one," and because Friends is a comedy show, this aspect of his character is emphasized. However, it is balanced out sometimes. Joey was the first one to deduce that Monica and Chandler were seeing each other, way before other people knew. Even then, Rachel and Phoebe only found out about Monica and Chandler's relationship by chance, whereas Joey actually figured it out from a mere eyelash curler. While this deduction was impressive for him, his funniest moments, and the show's funniest moments, still happen when Joey is being especially dim.
He essentially got the Homer Simpson treatment. Homer also began as a fairly normal suburban dad parody, but upon finding out how much laughter his stupidity could draw, he was written to get dumber and dumber. Television shows naturally grow depending on the actors' takes on their characters, audience reception, or plot lines that seem to come up naturally, and the program caters to the change. For example, Ross and Rachel's Friends ending wasn't the original plan, but as the show evolved, the story happened. Joey's dim nature was a selling point, and one that could be used endlessly for bits, thus that was the direction chosen for his character.
In later seasons of Friends, there is a certain level of self-awareness within the characters, almost as if they all know which roles to play. Even Chandler's catchphrases and mannerisms are parodied and mocked by his friends in the same way that they would be in real life by the audience. This extends to Joey, who seems to be dumb for the sake of being the dumb one. Yet, Friends mastered the art of never overdoing it, as is so easy to do with an incompetent character. Joey, in his growing dumbness, was always charming, hilarious, and lovable — so much so that he was the only Friends star to receive a spinoff.