Naturalist is a noun. It should not be used to modify another noun (aesthetics).
Naturalistic is an adjective. It may be used to modify another noun.
However, that said, I am sure your first construction would be (pardon the pun) natural and understandable, perhaps even more understandable than the second, because we get a picture of "naturalist" as a person, first, and then we are asked to think of his/her aesthetic values. The second construction, while correct, throws the emphasis onto the aesthetics, rather than the person, and then defines those aesthetics in our mind as naturalistic.
Rules should never get in the way of clarity.
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