I've used FastAPI, but haven't done a lot with Starlette directly. If you are building a full stack app, I can imagine the integration between FastAPI and Pydantic can make it easier to work with the data that you might want to render in the HTML that you generate using htpy?
htpy is just server-side rendering of HTML. Your routes are returning strings instead of structured data, so from the perspective of responses you're not going to be using Pydantic at all. That doesn't stop you from using it to validate objects you're passing around in your server, but I personally wouldn't do that because Pydantic can have a pretty hefty memory footprint. I've seen over-reliance on pydantic lead to plenty of OOMKilled errors.
It's a bit different for requests though. FastAPI will allow you to define your request schema (application/json or application/x-www-form-urlencoded) and validate it using pydantic, but starlette doesn't do that OOtB. It's trivial to implement though, and if it were me I would probably choose to do that rather than deal with FastAPI's inflexibility.
