Charles Dickens, like William Shakespeare, used character names to conjure up certain perceptions of the characters in the reader's (or audience member's ) mind. In the case of Hard Times, some of his satirical inventions are very easy to decipher; clearly, a teacher named "McChoakumchild" is not a paragon of progressive educational methods.
The use of "Gradgrind" as a surname continues Dickens' custom of portraying many in the merchant class as grasping and selfish, dedicated to "grinding down" workers without adequate compensation or any compassion. "Bounderby" of course brings to mind the classic epithet of "bounder," which means an unscrupulous cad.
"Sissy," on the other hand, should not be interpreted the way a modern reader might see it. As a nickname for "Cecelia," it's intended as a loving comment on this character, more evocative of, for example, a "sister of mercy."
"Sparsit," for a landlady who also provides board, brings to mind someone whose meals are likely very "sparse" and therefore not very nourishing or comforting.
Look for this kind of name usage in all Dickens' work, as interpreting it is part of the enjoyment of reading his books.