Hmm. From the context in the OP, I'm assuming graphics is combined with art style, so I'm going to give my answer accordingly.
Both are equally important to me. I started with Sims 2, and compared to other games I played, the graphics weren't exactly stellar so I used a lot of CC, which was fine with me, and I thoroughly enjoyed the gameplay. Without CC I may never have picked up the game at all. The appearance of my Sims was extremely important.
When Sims 3 launched, I couldn't stand the appearance of it and couldn't create a Sim to have the features that I wanted. But even when I made a "decent" looking Sim, they just looked awful to me. I also felt like the world graphics and Sim graphics were somehow misaligned and it was very distracting. It's hard to explain, but it contributed to my abandonment of the game very quickly. So in that case, the gameplay just couldn't save it. Also on that note, I have to add that I can't vote for gameplay in general, because a lot depends on the type of gameplay as well. There were aspects of Sims 3 gameplay that were lacking or invasive and it wasn't fun for my play style.
Sims 4, upon launch, was fun at first, and I definitely loved the graphics/art style immediately. I was, for the very first time in a Sims game, able to create a Sim Self just with the CAS demo and no CC. I was able to create likenesses. The worlds were breathtaking and I still love them. But I came to find out that the gameplay was stunted and the fun wore off quickly. But not quite enough for me to abandon the game because the basis of my gameplay was still there just enough to stick to it. But as they've added more gameplay to Sims 4 over the years, I think it has definitely improved more and more, and although I think it still has a ways to go, I can enjoy it in new ways that I couldn't in previous games.
But beyond the Sims games, my answer comes down to this:
Aesthetics are important depending on what I expect from the game itself. Anything that offers customization, or is designed around aesthetics, needs to have some kind of decent graphics and/or art style or it will fail to impress me.
Gameplay is always important but is not universally defined. So to each player and each game, "good" gameplay is going to mean something different. I play an MMO that has a player base consisting of Questers, PvPers, Raiders, Roleplayers, and musical performers. Each group wants something different from developers to suit their gameplay, and some cross over to enjoy one or more of a combination of play styles. So one player base might think the game is great as it is, while another thinks it's "lacking" in gameplay because it doesn't hard focus on their preferred enjoyment. So I think the same can be said for The Sims.
Essentially, when one person states that they want "gameplay", another person might picture that gameplay looking completely different. So in the end, I think it's very important to be as constructive and as detailed as you can when you request it, or the definition will be left up to the developers to interpret.