Curses are debuffs and do not have levels. Curse skills do. Part of what was changed in redoing curses was making the disinction more clear and removing stuff like this that blurred the two and caused confusion.
Curse debuffs have default values. If applied by a skill, the skill can override those values. You can see the default values for a curse in the reminder text of stats which apply the curse.
In general, the skills are balanced such that a level 1 curse skill applies the curse with the default values, and higher levels override them with higher values, but this may not always be the case for all curses.