You see it using maracas is very easy[simple hand movement up and down]but to use them skilfully such as timing the beat to a complicated piece .That would take a lot of concentration and effort. There is a certain skill to catching the beat of any rhythm. If you could say that maracas require no skill you will also be suggesting other key components to an orchestra are 'unskillful' such as percussion and cymbles.
Playing any musical instrument, it takes skill and time, even as something as Maracas it would take skill to play in a way that it will be enjoyably to hear. Anyone can pick up a musical instrument and play it, but it would sound horrible to a professional , but if you put time you will have the skill to play that musical instrument.
Maracas are not a skilled instrument because all you need to do is shake your wrists to pretty simple beats. With say a guitar or a piano, you need to learn notes, scales, timing, theory, beats, fifths, fourths, octaves, sharps, flats, key signatures and more, and it takes years to learn these things. You can learn to play the maracas in only seconds so no it is not a skilled instrument.