Is low power mode in iOS good for battery?
Depends on what you mean by 'health.' If you mean time before you need to recharge, then yes - by reducing the background services, the battery will last longer until you need to recharge it. If you are really strapped for energy, turning on airplane mode or just shutting the phone off entirely until you absolutely need to use it.

However, if by 'health', you mean the number of charge cycles the battery will last, that should remain unaffected.