If by "almost as soon" you mean "a generation later."
Calling her a Caligula is probably overblown. That said, she falls under the category of a lot of tyrants such as Vlad the Impaler or Oda Nobunaga. Not nice, not good, but potentially beneficial to the country. And the persecution of Christians was
targeting legitimate enemies, since the Christians weren't too thrilled with her worshipping the old gods either.
That's how Africa works, really. Thrones are propped up by a mountain of skulls. Ranavalona defended her nation the only way she saw possible (and whether or not you think colonialism is a good thing, it's pretty much a given that a sovereign has a right to defend her territory), and she did a pretty damn good job at that.