Naamplao is right - people are being confused. I'm sorry I started this - it (mundane/mundo/of this world) just seemed a rather neat play on words.
The expression 'not of this world' is rare. The BNC, in all 70 registers (I don't know how many billion words that is), has only 14 occurrences - many of which come, directly or indirectly*, from the King James Bible:
1 ADC kept alive a sense that Christ's kingdom was not of this world. Other forms of holiness --; that of the virgin and the
2 B1F of a ship. But this sound is not of this world for other sounds are heard distinctly and cause this sound to die
3 CAG electoral game for all it's worth 18 Not of this world Our politicians have their heads in the sand, says Fred Halliday
4 CAG are closer and the engines are slowing … Not of this world British politicians are still burying their heads in the sand, says
5 CCL to the Kingdom precisely because it is "not of this world" and presents such a challenge which arouses hostility. "Happy
6 CDW those intriguing words, "My Kingdom is not of this world." In answering this question he was contrasting his own Kingdom
7 CDW but he said, "My Kingdom is not of this world." It is against this that we must judge Jesus's
8 CLN not to obscure the glory of a Kingdom not of this world. The abolitionist appeal in parliament revealed subtle differences. In the
9 CRE knows," Godolphin said. "Something not of this world, clearly." "What did it want?" Alice
10 EDY who beckoned his followers to a kingdom "not of this world". Biblical scholarship during the last two centuries, however,
11 FP8 tradition that taught that "my Kingdom is not of this world," and that states are usually just "great robber bands
12 HA4 he probably won't notice. He is not of this world, you know, in some ways we're so far apart
13 HAK ," he said. "They are not of this world. And some of them spend more on lunch than the workers
14 HPW doors of heaven. " My kingdom is not of this world " was not regarded as releasing the pope from an active role
* 1,5,6,7,8,10,11, and 14 - and some of the others too, possibly (to the extent that Christian morality underlies some of the other cases).
Sorry -
b
The original question though was NOT about the expression "not of this world", which I do agree is a very rarely used expression.
Rather the question was about "out of this world" meaning fantastic, very good.
"Gee ma, this meal is out of this world! Make it again soon, please!"
"out of this world" has 1,810,000 Google hits and I don't know about your part of the world Bob, but around here it is still a reasonably common expression.