In your first example, the phrase is being used as a proper noun. It is the name of a specific principle and should probably be capitalized, quoted, or even hyphenated for legibility.
The Montessory principle of Simple to Complex says...
The Montessory principle of "simple to complex" says...
The Montessory principle of simple-to-complex says...
(As an aside, "The principle of simple to complex" is the full name.)
Your second example is a title. Titles often sacrifice correctness for style or conciseness because they have a limited number of words in which to convey the general meaning, subject, feel, or context of the body they title. Of Mice and Men springs to mind.
In general, from simple and to complex would not make sense on their own. However, the prepositional phrase from simple to complex describes a range.
One case where it may be useful is the following sentence.
I enjoy solving simple and complex puzzles.
This reads as if I enjoy solving two types of puzzles: simple ones and complex ones. Obviously, that is not what I mean to say. I enjoy solving all kinds of puzzles despite how simple or complex they may be.
I enjoy solving puzzles of simple-to-complex difficulty.
This expresses the fact that I enjoy solving a range of puzzles, but still misses that the range is not particularly important to me. It says that I enjoy solving puzzles in the range.
I enjoy solving puzzles of all difficulties, from simple to complex.
This expresses my meaning and is reasonably clear to a native speaker. The adjectives are in a non-standard location, but the sentence is still correct because the prepositional phrase is a range qualifying the difficultly of the puzzles I enjoy solving.