Having an iPad continuously powered for extended periods is not an issue - and will cause no harm - as this is common practice in POS (Point of Sale) and Kiosk applications.
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. 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
Independent studies have demonstrated that restricting charging to only 80% of maximum has little if any impact upon the overall longevity or health of the iPad's battery.
Be aware that your iPad power is shared by the external power source and the iPad's battery. While the fully charged battery will effectively be being "float charged", the battery will still provide additional power, where needed, for surge demands.