No, Foster’s Home for Imaginary Friends is not set inside Frankie’s head; this is a fan theory and a plot point explored in the episode “Destination Imagination,” where the character World creates a mini-world inside a toy chest that is mistaken by some as being a representation of Frankie’s mind. In reality, the series is set in a house for imaginary friends who are adopted by new children.
The Theory and the Episode
Destination Imagination:
In this episode, Frankie releases an imaginary friend named “World” from a toy chest. Mac, Bloo, and the other friends fall into the chest and discover a world created by World, which has a miniature version of Foster’s.
Fan Misconception:
The fan theory that the entire show takes place inside Frankie’s head stems from misinterpreting World’s creation. The miniature world in the toy chest is World’s own creation, not a reflection of Frankie’s mind.
The True Premise
Foster’s Home:
The show’s actual premise involves an orphanage for imaginary friends. Children outgrow their imaginary friends, and these friends are sent to Foster’s until a new child adopts them.
Main Characters:
Frankie is the granddaughter of Madame Foster and works to run the daily operations of the orphanage.

Mr Conspiracy Manhattan aka Victor Sage aka Mr Conspiracy Manhattan