yeah, animation is a form of modern art. A legitimate genre of mod art emerged in the 1960's called "kinetic art" around the same time that Warhol was trying to establish industrial & commercial design as art (I met him once).
At that time, the kineticists were only sculptors, but, of course, there was entertainment animation pioneered by such people as Walt Disney. But animation became a serious art form with the French as "bandes designee" or design strips.
As far a legitimate art-forms, it goes back a long way, for example, Michelangelo used cartoons as the first step in such works as the ceiling of the Sistine Chapel. And Rubens used to do designs on canvas in yellow ocher (in those days a form of tan clay), and let his students color the work in (as long and they could color inside the lines, of course).
But the ultimate purpose of art was discovered by such as da Vinci, who, through the stimulus of visual art, developed his visual cortex, hence being able to think with the eyes, and also became a significant inventor (which I call The da Vinci Factor). I have done some experiments with classical abstract anime using the computer ...