Is Santa Muerte an official saint?

No, by the Catholic definition of "saint"

For the Catholic Church, a saint is always a human being who lived the Christian virtues heroically, in communion with God, and who after death is formally canonized through an official process. Santa Muerte meets none of these criteria: she isn't, and never was, a person — she's the personification of a concept, death itself.

That's why Catholic publications like Boletín Salesiano are explicit in noting that the Church "opposes giving death the title of saint," precisely because that title is reserved, in its doctrine, for human beings.

So why is she called "Santa"?

The name reflects popular devotion, not official recognition. In the same way a community can venerate a figure as a saint long before — or instead of — any official canonization process, devotees use "Santa" as a title of respect and closeness, not as a claim of canonical status. This distinction — between official recognition and popular devotion — is common throughout Latin American folk Catholicism, not unique to this devotion.

Is there a comparable, officially recognized figure?

Not one that's directly comparable. By all available evidence, Santa Muerte remains entirely outside the official Catholic canon of saints, and there's no canonization process underway or planned.

Related: What does the Church say? · Is it a sin to venerate her? · Is it a religion? · Who is Santa Muerte?