The origin of the phrase is from a review of Clive Barker's Weaveworld which said it was 100 pages of horror and 600 pages of hobbitytosh.
It is reference to the more flowery prose used within the Hobbit series of books, and latterly films, and has subsequently applied to films/scripts of a similar bent. For example, to Mark's ears, the following sentence regarding the sword belonging to Aragorn, is full of Hobbitytosh.
Before Isildur was killed in the second year of the Third Age in the Disaster of the Gladden Fields, the shards of the sword of Elendil, killer of Sauron of Mordor, were rescued by Ohtar, squire of Isildur. He took them to Arnor, where they passed to the next king, Isildur's son, Valandil.
Mark often gives examples of Hobbitytosh by using phrases such as "The Thring of the Throng" or "The Flibbertygibbet of Hoohar".