If you’re referring to the performance overlay, the plot on the left is the app frame rate, not your headset’s refresh rate. The plot does say Hz, but all the unit Hertz means is “cycles per second.” It might technically be more correct to label it fps on that plot but they mean the same thing in this context.
The real problem you’re having is that you aren’t able to get an app frame rate better than 30fps, which is then being interpolated by ASW up to 90fps (for the 90Hz refresh of your HMD screens). This means you already have way too little performance headroom to run smoothly, because it won’t automatically step down to 30fps until the GPU is choking and probably at 99% utilization. Start by lowering the settings of Link (the render resolution and the refresh rate) in the Oculus PC software, and set supersampling to 0. Also dial back the render resolution in MSFS, too. Only once you’ve found a starting point where your system can keep up, can you start adding other stuff into the mix to see if it kills performance. Lastly, if you’re using Quest with Link, open up the Nvidia control panel, go to the per-app settings, find MSFS, and change the Vertical Sync setting to ‘fast’. If you don’t do that, even after dialing back your settings you will still have a persistent stutter that will not go away.