Right but it's not your responsibility to direct the UI. Your responsibility is backend services, possibly at a stretch, the architecture of apps as they relate to using those backend services, but it's most definitely not the aesthetics.

What if the app is headless and the LLM tries to stick a UI on it? What if the app is a TUI and the LLM gets stuck on terminal fonts? What if my UI aesthetic is grungy hackercore and the LLM tries to make it look like every other Tailwind website?

This criticism/feedback is less about what's written, and more about why it was deemed appropriate. You're getting direct input to the development process of your customer's products, and you're using that responsibility to... make pointless comments about design?