why would that be a bad idea? to my uninformed brain that actually sounds interesting

With the modern short wavelength UVC LEDs, this is considered a promising new technique [1].

With longer wavelength UV, which is less absorbed by the cells, the light would penetrate deeper into the tissue and cause much more DNA damage. Still, broadband UV irradiation of throat, nose, and ear was a mainstream treatment in the USSR.

[1] https://www.nature.com/articles/s41598-023-49745-3

I remember Trump mentioned it in the beginning of corona as one of the discussed ideas and everyone laughed.

It kinda is laughable for a virus that lives in your lungs and blood.