Hi, while you may get answers here, this question would be a better fit for our sister sub, /r/AskAnthropology. Consider x-posting for their input

Charlemagne? That's barely over 1000 years way. Not enough time for evolution to change our brain so deeply anyway. Besides, it is not so hard to pick up how to use a smartphone. Most modern humans have problems grasping ancient greek philosophy, most modern humans wouldnt be able to figure out acient egyptian or babylonian astronomical and calendrical calculations (one of the harderst area of study even for modern historian of the ancient world). Humans havent supposedly changed in the last 30-40k years at the very least.

Short answer is yes. If you started handing past man modern technology they'd suffer culture shock but a medieval man is the same species of human as we are so they had the same intellectual capacity as us