my iphone 6 get hot when i am using cellular data, is it normal?

Depending on the network connection (LTE or 3G) and the period of time you are connected to the cellular data, the phone might get warmer than usual. But, if it getting too hot to touch, there is something wrong, and it has to be checked by Apple or an Apple Authorized Service Provider.

iPhone - Contact Support - Apple Support

Maybe also take a look at this article:

Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support

Heating Up is usual when you use Cellular Networks.

Try using WiFi & check Phone gets too Warm.If not try Resetting Network Settings & Update to Latest iOS.

Normal Heating Up doesn't affect on Phone.