To find out which iPad mini you own, take a look at the model number. It’s located on the back cover, and starts with an A, followed by four digits. A1538 or A1550 would be an iPad mini 4. It’s camera’s features are almost the same as those of the iPhone 6.
A different number belongs to iPad mini, generation 1 - 3. The camera on those devices have 5 MP, which could be the reason for grainy images. It’s not possible to do anything against this, afaik. You can’t change the technical specifications of the camera.