Expanding upon Rudegar’s comments, you have no reason to be concerned.
Assuming that you were using the Apple supplied Power Adapter, or a good quality alternative, the iPad will automatically regulate and optimise charging of the battery. iPad and iPhone are actually designed to be left connected to their Power Adapter for extended periods - as, in addition to charging, other background activities occur whilst connected to the Power Adapter. Overcharging is not something that you should generally be concerned.
While the iPad is connected to external power, is locked and connected to a WiFi network, the iPad will often complete background housekeeping tasks - such as automatically backing-up and synchronising to iCloud or downloading an update in readiness for later or automatic installation.
As for the battery indication, the observed behaviour can vary for a number of reasons other than the more obvious causes - such as CPU load/activity and screen brightness. In early days, or after a system reset, the battery indicator may recalibrate itself - leading to a perceived change. Unless you experience a significant drop in actual battery endurance in-use, minor change in indication is not a cause of worry.
Tip: Disable the battery percentage in iPad Settings. With this reading disabled, the habit of stressing over the % reading is removed. You still see the relative battery level.
Settings > Battery > Battery Percentage - set to OFF
I hope this information and reassurance is helpful. Enjoy your new iPad.
