Is 90°C Normal for a 9800x3D Under Load with AIO Cooling?

Before anything else, make sure you removed the protective plastic from the AIO cooler before installing it on the CPU. If you did, it shouldn't be running close to 90°C at all under regular conditions.

What brand and model is your AIO cooler? If it's a 240mm, hitting those temps is pretty normal for stress tests, but with a 360mm, that's on the hotter side yet still acceptable since the max junction temperature is 95°C.

I have a 9800x3D with a Thermalright Phantom Spirit 120SE and I was also hitting the 90s in Cinebench. After undervolting my CPU, I dropped my temps by about 10 degrees during stress tests, and in gaming, I rarely hit 70 degrees. It's pretty normal to see those highs during stress tests, but as long as you're not reaching those temps while idle, you should be just fine.