Comparative adjectives express a higher degree of a quality. For example:
* big (positive degree) -> bigger (comparative degree)
* cold (positive degree) -> colder (comparative degree)
"Suddenly" is an adverb describing the manner in which something happens, meaning it happens unexpectedly and without warning.
You can't compare "suddenness" in the same way you can compare size or temperature.
If you're looking for a way to express a degree of "suddenness," you might use phrases like:
* more abruptly
* more unexpectedly
* more rapidly
For example:
* "The door opened more abruptly than I expected."
* "The storm hit more unexpectedly than anyone could have predicted."