I would say so.
The Gimli on dead uruk is clearly a small diorama.
In your case you wanted to show goldberry by the river bank.
Of course if you enter her in the painting the focus will be on the paintjob, and the base itself will be rather irrelevant. While in the diorama competition the painting, while still important, will probably lose to more elaborate ideas.
Guess you could enter it in both