Congratulations on making your first game in Pico-8. It's not easy making your first game, and even harder trying to make it for a jam. So kudos to you! The shooting gameplay felt a little bit clunky, the collision code probably needs tightening up, it would be nice to have some more sound effects too, like confirm/cancel sound effects on the downgrade menus. That's the (minor) negatives out of the way.
I loved the idea of making the robot more fuel-efficient whilst lowering performance, really cool way of linking progression with increasing difficulty--something a lot of RPGs struggle with. I loved the lore of the game, the idea of being some old, forgotten robot stomping around the wasteland after the war is over is just great.
The graphics were really nice, I liked the little enemies running around and the different animations/poses of the player character. The design of the player character was really cool.
Awesome first game, and awesome jam entry. Respect!