Van Gogh's Pieta is not a great painting, and it definitely is not an example of a superior pieta. Much of Van Gogh's other paintings, particularly his early work, are equally unimaginative and lacklustre. However, I consider some -- especially the ones he painted shortly before his death -- to be masterpieces.
In some of his paintings, Van Gogh succeeeded in capturing and conveying the aliveness of Creation. Everything he depicts on his canvases in such paintings -- the sky, the stars, trees, animals, plants, rocks, hills, fields, buildings, entire landscapes -- teems and overflows with life, and Van Gogh triumphantly manages to portray the dynamism and energy of Beings intersecting and interacting in Creation. Everything swirls and flows and moves and relates in dizzying yet somehow serene kaleidescopic beauty.
Thus, I am unsure how great a painter Van Gogh really was, but I consider some of his paintings to be truly great.
Note: Van Gogh was also diagnosed with "acute mania and generalized delerium" when he created the paintings below (1888-1890). Pieta was also painted during this time.