First we will start with Grandpa Abraham Simpson: He now seems senile but throughout the Simpsons there are flashback of occupations grandpa Simpson had. He was a fighter pilot and an amazing pianist among other occupations, which require at least an above average intelligence. Grandpa is the only one who isn't currently smart because of his old age.

Marge Simpson: She was an excellent student and had a chance of having a promising career but decided to marry Homer and become a housewife. She decided having a family made her happier than having a career.

The main concept of why none of the Simpsons really embrace their intelligence except for Lisa is "Happiness over intelligence"

Homer Simpsons: Homer may seem too stupid to be a genius but in Season 12 episode 9, Homer discovers he has a crayon stuck in his brain which he put there when he was a child. The crayon is removed and homer becomes the worlds' smartest man to had ever lived. Homer was very smart but was driving his friends and the city away because he was being extremely obnoxious. In the end he stuck a crayon back in his head. He chose to be "happy and a moron" over being "a genius and miserable".

Bart Simpson: Bart decides to follow the same path as his father at a much younger age, without having to stick a crayon in his brain. One of the episode shows Bart to have had good grades when he was younger but then his grades started to decline. This episode blames it on "The Simpsons Gene" but the Homers crayon removal proves "The Simpsons Gene" wrong. Bart grew up with his father being an idiot but happy. This proves to him that ignorant is indeed bliss. He follows his dads' path to be an idiot but happy. Bart's intelligence show when he creates pranks, which are original and complicated. Even when he wants to do better at school in his subconscious he doesn't want to be intelligent. He continuously teases Lisa about being a "nerd" but he does this out of love. He wants her to make the same decision he and his father made.

Lisa Simpson: Now Lisa is extremely smart and shows it, but she is always stressed and too tensed to have fun. She is the saddest character. In one episode she is so depressed she is prescribed "happy pills". Lisa despite not being fully happy she can't let herself be dumb. Lisa is the only one who doesn't choose happiness over intelligence.

Maggie Simpson: Now Maggie is an infant, she is the youngest in the family. She is too young to decide whether to choose a path of happiness and being dumb or like her sister be smart but sad and stressed all the time. However, even though she is still an infant on numerous occasions she shows she is very smart, she helps save her fathers life multiple times. She will later make a decision with her two siblings as examples to guide her.

Thanks for reading:)