Well, my game scored mediocre on graphics, so I'm surely not an expert. And that's while I used professionally made, commercial graphic assets for most of my game. Have you checked the games with the highest scores in the graphics category?
I definitely don't want to discourage you; quite the opposite, I applaud your efforts. A few years ago, I wanted to create a 3D shooter game - didn't achieve to release anything. 3D with real people in first person view is the most difficult environment for graphics, in my opinion. 2025, we're looking at hyper-realistic images, motion capture, AI supported workflows; expectations can be very high. On the other hand, pixel graphics can be appreciated well if they achieve a consistent and polished style.
For the graphics in your game, my first suggestion would be to add basic animations to people. In general, you want to avoid the uncanny valley, and this includes movement as well. It's not necessary to aim for high resolution but does the game look natural to you when you step away a few meters from the screen? Do lighting and shadows fit? Buildings might look all the same but living beings have variation, also idle movement. As I wrote before, the most important next step to me is to keep polishing the game. Good luck!