It is not unusual for an iPad to become warm when being used - more so when using some types of iPad case. High screen brightness and/or CPU intensive Apps will also create more heat - and in some situations will cause an iPad to automatically take steps to avoid damage to its battery or electronic components.
Are you using your iPad outdoors, or in very bright ambient lighting? If so, the iPad may brighten its screen to maximum, adding to generation of heat. Moving indoors to more subtle ambient lighting conditions can help, as can removing a fitted case from your iPad.
Ensure that the iPad has good ambient ventilation. Avoid placing the iPad on soft furnishings - as this added insulation can cause the iPad's internal temperature to soar.
If your iPhone or iPad gets too hot or too cold – Apple Support