"Sparkle" has been used as an adjective for "special snowflake" and "Mary Sue" qualities. For example, I've seen the term "sparkledog" used a lot over the years to describe the plethora Mary Sue canine OCs. In this case, "sparklegender" refers to those nounself pronouns, like bunself and faeself, since the majority of them are most likely being used by people who want to be super special.