1967 was a very different year than 1954. Even when authors or artists were not explicitly trying to make a statement, the cultural difference creeps into everything, even whimsical children's books. The mushroom planet books started in 1954 when Eleanor Cameron wrote The Wonderful flight to the Mushroom Planet when her son asked her to write a book about space travel.
Time and Mr. Bass is the last of the five mushroom planet books, and while the book is not "tacked on" to the series, it is a far cry from the gee-whiz-bang spaceship world of Voyage to the Mushroom planet. The boys are still here, and so is Mr. Bass. But instead of Mr. Bass being a scientist, his wonderous powers are now revealed to be somewhat magical, and related more to his status as a descendent of Welsh Bards. This book mostly takes place in Wales, with brief side trips to London and Basidium. The plot of the book centers around ancient evil forces and the search for the bones of Mr. Bass' ancestor, a legendary bard.
This book is dark in some places, in such a way that any children who wouldn't be ready for the darkness wouldn't be able to recognize it, and so wouldn't be disturbed by it. It also is quite mature psychologically in places, as when it tells of how a relic of the Mushroom people gives earth people who have parts of it strange delusions and obsessions with things reminescent of the mushroom planet. Of course, the book (and the series) does end on a happy note, as Eleanor Cameron brings her story arc to a close.