First of all, no. This is nothing like the Ed Edd and Eddy theory (which still freaks me the heck out).
There's a theory that the popular show Adventure Time is all just real world Finn's dream.
Which would make sense.
I mean, a young boy fantasizing about magical adventures starring him and his magical talking dog ain't uncommon.
Also, just look at some of the characters!
An ice-themed wizard. Sounds like either the imagination of a kid who's afraid of Santa Claus or the imagination of almost any kid. Plus, he kidnaps princesses.
A princess made out of gum. That's all.
People made out of candy. That's all.
Plus, the episodes Incendium and Go With Me could just be the over-exaggerated escapades of a little boy who recently just got into the dating business.
And don't get me started on Reign of Gunthers!