I think the complaint is about attribution. If a friend takes you to the park to watch the cranes and ducks, it would be a stretch to call them a great artist for facilitating that potentially sublime experience, because anyone could have taken you. Similarly Pollock's paintings may be aesthetic, but if they are easily imitated, it's hard to argue the works themselves make their author a great artist.

To be fair to Pollock, he had the creative insight to make his work in a less permissive period of art, so it's fair to argue he was a great innovator or artist on that merit, but it's a different type of achievement vs 'timeless' pieces of art like (imho) Bernini.