I recently had back up camera installed at the shop and everything works fine but I noticed that when I opened the tail light cover, the shop guy drilled the hole and connected ground wire with screw.
Is it safe to install like that as long as it works fine?

 

While I've never seen it's being done like this in theory it should be fine.
As long as the screw is driven into bare metal or any paint was removed for a proper connection.
You may also want to apply silicone caulk or similar coating to prevent any corrosion.

 

@yyj3869 ,

I prefer to use a machined bolt and a nut for a much better securement than a screw. That’s my preference.