iPhone as a hot spot Is it bad for the phone?

Do you mean harmful to you or harmful to the phone? It will drain the battery pretty quickly if you are concerned about the phone. It won’t be harmful to you.

No.

“Harmful”? It depends on the degree of harm you’re talking about.

Is it harmful to walk under a high voltage power line? Or to receive an X-ray? Possibly- infinitesimally.