Accepting equality is probably desirable, but as the test for ascending is first it will have precedence. If we accept both we probably need to defer the decision to find which it is. Otherwise there will be an inconsistency like:

  >>> monotonic([2, 2, 1])
  [[2, 2], [1]]

  >>> monotonic([1, 1, 2])
  [[1, 1, 2]]

As a separate detail, zeros are regarded false in Python:

  >>> monotonic([1, 2, 1, 0])
  [[1, 2], [1]]