The creator was a fetishist, but intended for her to be that elegant, matriarchal figure, not a pin-up model. H. G. Peter was instructed specifically to make her this way, avoiding aspects that would make her overly sexualized. There are some earlier designs than this one that featured a shorter skirt and a more stylish top, that Marston rejected as being too sexual and similar to characters like Fantomah.
Marston based the character on his mistress, Olive Byrne, his ideal woman, who was classy, elegant, liberated, intelligent and beautiful. Marston was also a radical feminist who believed in the superiority of women as gentle, matriarchal figures. And though he did like bondage and having Diana tied up, he also used it as a theme of having her break out to represent liberation, which was based on Olive's beliefs.
tl;dr Marston was very specifically opposed to women as traditional sexual objects and even wrote in his introduction to the character that "girls don't want to be girls" because they are often written as objects for men instead of their gentleness and grace being shown in a positive and inspirational light.