Corey, are you sure it's the liquid diet? There's a phenomenon many people experience after surgery. Whatever goes wrong, any little headache, any change in digestion or bowel movements, any weakness or fog, they attribute to the surgery. This has the potential to be dangerous. There might be an entirely coincidental problem causing brain fog or weakness.
I didn't feel tired or foggy. I don't think it's normal.
I did feel shaky, but I knew that was because I went off sugar, and that's hypoglycemia, and that's common. You can improve it by drinking small amounts of fruit juice, or even stuff like Jell-O. I remember feeling weak and filling a teaspoon with sugar, sticking it in my mouth and letting it melt into my stomach. Often, that worked for me, and it was the only actual refined sugar I ever allowed in my diet. I knew it was low blood sugar, and I knew my body wouldn't feel better until I brought the sugar level back up.
This isn't something a diabetic should do, but IMO it's harmless for people experiencing the tremendous shift from sugar to no sugar.
Ask your nutritionist, but listen to your body, too. Sugar may not be something you should have yet, but neither is brain fog. It's an important medical question and it may be a coincidence. Get a professional opinion and tests for blood sugar, etc.