SpongeBob takes place in the late 1950s and early 60s, but it’s not explicitly stated
The styles of the clothing and hair look very late 50s and early 60s. Almost every female is wearing a dress except Sandy.
Some of the soundtrack music sounds very similar to Beach Boys and other artists. Ripped Pants is a parody/soundalike of “Be True To Your School.”
Flower clouds evoke the “Flower Power” movement and flowers everywhere during this decade.
Stephen Hillenburg grew up in the 60s and it would make sense that he would base it on his childhood.
“Square” was popular slang that meant uncool or not popular, hence why SpongeBob acts like the way he does.
There was nuclear testing in the Pacific around this time, again, it would make sense for the creatures to be talking and doing human things. Explains the “war” that Mr. Krabs fought in.
Random explosions=nuclear testing?
The cars and the technology are very old fashioned (exception: the computer in the “we have technology” episode)
Even Karen is somewhat old fashioned to me, Plankton’s lair reminds me of a control room for the original “computers.”