An iPad will come to no harm if connected to its Power Adapter for extended periods - any more than being actively used in this configuration will cause degradation of its battery.
When connected to a Power Adapter for an extended period of time, such as when used in Kiosk or PoS (Point if Sale) applications, iPad will automatically change its charge management strategy - and reduce the maximum attained 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. As soon as a “normal” charge/discharge pattern is resumed, charge to full capacity is restored.
Using the iPad whilst connected to its Power Adapter can actually extend the working life of an iPad - by reducing the accumulated charging cycle count. For an otherwise healthy iPad, it is the number of charging cycles that will ultimately determine the working life of a battery operated device.
An iPad battery is rated to maintain 80% of its initial charge capacity of 1000 full charge cycles - a full cycle being defined as any combination of charging that totals 100% (e.g., 40% + 30% + 20% + 10%).