I'd echo others: unless you see something that is a clear and present danger (e.g. hey look, a passenger is trying to open cabin door ... there is fluid pouring out of an engine ... you get the idea) then I would not get involved in enforcing the rules on board. As @
canadiancow points out you may not know the rules entirely (rules on devices, headphones, etc. are changing a lot and differ between airlines) and as @
Badenoch points out the downside more than offsets the upside in my opinion.
I was on a flight 2 weeks ago where the FA asked the passenger across the isle from me to put away his laptop and fold up his tray table before we took off. He complied, waited until she went and sat down, and then pulled it out again and sat it on the tray for takeoff. Some people just don't care, but I'm not going to get involved in enforcing the airlines rules unless I think my personal safety is at risk.