Is Norse and Celtic pagan art creepy?

Yes. But also NO.
There are many different types of paganism and I follow the Heathen path or it's called Asatru. Art based on the Norse and Germanic heathen gods and goddesses, is gloomy, to reflect the myths. This is because of the dark landscape with severe Winters, dreary weather, little sunlight, and it's always colder in the north. Therefore art which is featuring the stories from the Eddas and other Germanic legends, even Celtic myths, will always look darker. This doesn't mean there isn't colour to the masterpieces. Some paintings bring vivid dreamy pictures of the myths and legends.
As for "creepiness" well yes because it isn't meant to be nice and fluffy.

Howls ^^