While Cousin Stanley does share some visual similarities with SpongeBob, he's actually a later addition to the show. The original concept for SpongeBob was a much simpler design. It was a yellow square with round eyes and a big, goofy smile, more like a cartoon character from the 1950s.
It's important to note that:
* The idea for Cousin Stanley came after SpongeBob was already established. He first appeared in the episode "Can You Spare a Dime?" He's a sort of comedic homage to the original, simpler design of SpongeBob, playing on the idea of the "generic" cartoon character.
* SpongeBob's design evolved throughout development. The final version of SpongeBob is the result of many iterations and refinements by the creators, with the goal of finding a design that was both visually appealing and expressive.
So, while Cousin Stanley is a fun reference to the original SpongeBob concept, he isn't the original idea itself.