If that is a kill A Watt use it on a extension cord.
Not good to let a Kill A Watt digital tester ride on a vibrating generator.
Also try the HZ meter you have at the end of a 25 ft or so cord with about a 100-600 watt load on the gen and sometimes just moving the tester away from the gen on the end of a cord will get a more stable reading. Some cheap testers are not shielded internally and will pick-up electrical radiation from the portable emergency gen.
and Yes them gen's sine wave can have harmonics and noise, but a good tester will usually read correctly.
Do not try to operate any electronics from them kind of gens.
Inverter gen's are the clean ones usually because they use addition electrical filtering clean the voltage generated.
Your gen is most likely ok since your cheap tester reads House HZ ok at 60.
Forgot to mention that if the you do not HEAR the engine speed change very much when YOUR CHEAP TESTER HZ is indicating 70-90 HZ and the voltage is not changing very much it's your tester.
China is talking to you AGAIN through your tester.
Also:
Safety Warning about cheap electrical testers:
Be aware of the really cheap testers. They can become a fireball/bomb.
especially when attempting to read a voltage over 150volts.
You can review the CAT (category safety ratings) rating of electrical testers and eq at this link and on-line. If you don't see a CAT rating spec for your tester do not use it at all.
https://www.digikey.com/en/blog/what-ar ... ty-ratings