Is it good to keep iPhone always on low power mode

No harm to your iPhone, if you can cohabit with the limits listed in this support article:--> Use Low Power Mode to save battery life on your iPhone or iPad - Apple Support

Regards

Giulio

Pls Note That Having Low Power Mode On All The Time Reduces Power, Disables Apps, And Suspends Many Features. If You Don't Care If These Features Are Turned Off There Is No Harm Keeping Low Power Mode on Constantly Or May Be Even Permanently.Also, Some Task Might Not Work Until You Turn Off Low Power Mode, Or Until You Charge Your iPhone/iPad To 80% Or Even Higher,However Turning On Your Phone Low Power Mode Will Make Your iPhone/iPad Last Longer Before You’ll Need To Charge It Again

It won't hurt, but it also won't help.