AMD Fluid Motion Frames worked like a charm on Asus ROG Ally X but let me tell you, AMD Fluid Motion Frames 2.0 is even better on Asus ROG Ally X.
AMD FMF 2.0 was previously in tech preview, and you had to force install it but that has now changed. With the launch of Adrenaline 24.9.1 driver update, you can simply update the driver to get the Fluid Motion Frames 2.0 ready to go.
Link: Adrenaline 24.9.1 Download

How to Install AMD Fluid Motion Frames 2.0
Update to the latest driver version and you will be able to use AMD Fluid Motion Frames 2.0. However, there are a couple of things to keep in mind.
You need to V-sync in your game in off and you play on Fullscreen or Exclusive Fullscreen mode.
AMD AMD Fluid Motion Frames 2.0 is enabled for each game individually through the handheld’s Gaming settings.

Select the game and you will see the AMD Fluid Motion Frames 2.0 option listed in the settings. Enable AMD Fluid Motion Frames to start boosting frame rates in your Asus ROG Ally X games.

When you enable Fluid Motion Frames 2.0 you see the option set Search Mode and Performance Mode. Leave the Search Mode on Auto and Performance Mode on Performance or Quality depending on the results you see in the game.
Usually, Performance gives the best results but depending on the game you can test Quality as well.
If you’re using Linux on Asus ROG Ally X then you won’t be able to use Fluid Motion Frames 2.0.