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Using the glossary of art movements, schools, and styles compiled by Joelle Steele (http://www.joellesteele.com/artmovements.html), perhaps the best fit would be folk art, or "naïve art" or "outsider art."
Folk Art. Art created by people who lack formal training, but follow an established tradition of craftsmanship and style, often regional in character. The art is generally imaginative and simplistic, lacking in proper perspective, and portrayed with bold or non-natural colors. Also called "naïve art" and "outsider art." Artists of this genre include Grandma Moses, Cook, Henri Rousseau, and Hirschfield.