GPU usage and FPS are not totally dependent. You can have 100% GPU usage and 9,000 FPS, or 40% GPU usage and 25 FPS. It really depends on what the bottleneck is and what you are doing. The GPU usage has nothing to do with the performance, but rather the utilization. Which implies the efficiency, but that can't be computed without also knowing the performance (or FPS). But none of that matters unless you consider what you are rendering. For example, rendering The Last of Us 2 on an Android phone at 60 FPS with 100% GPU usage would be rather good. Or if you were rendering Super Mario Bros 1, even at 40% GPU usage, it would suck if it were 15 FPS. So there is a lot to consider.
However, I have gotten really good performance on 2D (and even 3D) with Godot on Android, so if may be that you made a mistake, or that your game is not properly optimized. You can use the profiler to check this, though it is kind of complex to decipher. More likely you will just need to do more tests or tutorials so you can familiarize yourself with how Godot works, so you can learn how to optimize it. But this comes with experience.