Is a portable battery damaging to a ipad or iPhone?

Having an iPad continuously powered for extended periods is not an issue - as this is common practice in POS (Point of Sale) and Kiosk applications.

Whether you are using an AC-powered USB Power Adapter, or portable PowerBank battery, makes no difference whatsoever.

iPad will optimally manage battery charging - and may automatically change its charging strategy by reducing the maximum battery charge level to about 80% of maximum capacity when connected to external power for extended periods. This strategy is intended to extend the life of the battery by reducing chemical ageing that occurs when the battery is continuously maintained 100% charge. When a “normal” charge/discharge pattern is resumed, charge to full capacity is restored.

More information about iPad’s battery management can be found here:

About iPad and iPhone Charge Management Feature – Apple Support

It's no more damaging than using a wall powered USB charger to charge the device. The power source is a battery rather than an AC adapter. The iPad or iPhone will not be able to tell the difference.