When Low Power Mode is on, your iPhone will last longer before you need to charge it, but some features might take longer to update or complete. Also, some tasks might not work until you turn off Low Power Mode, or until you charge your iPhone to 80% or higher.

Is it bad to charge iPhone low power mode?

No it will help the device charge faster.

Is it OK to charge on low power mode?

Low-power mode is a better alternative when your phone's battery is on its last legs and you just need to make it to the next charge, or when you know you'll be away from power for a prolonged period and you want to stretch the phone's full charge as long as possible.