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.
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