There's a difference between reviving and being immortal here. Digestion is probably extremely risky for whoever is doing the eating. It's not that shit needs to revive, it's that all the tiny particles that your body is absorbing are still alive and regenerating. They'd essentially turn into your cancer until they destroy you by growing out of you.
m8, magical fantasy creatures work by magical fantay rules, and in those rules, "eaten and digested by a monstet = gone", not "regenerated back mixed into the eater's atoms like if you were part of Isaac Asimov's novelization of
Fantastic Voyage" (which BTW is a book I highly reccomend, in which it's explicitly mentioned how that would happen with the miniaturized submarine's atoms even if digested by the antibodies - whih is a huge plot hole of the movie that Aimov's book takes pains to actually solve - no spoilers in how).
Unless she is actually poisonous (and we know mermaid flesh is not poisonous, so my money is on "no"), eating her would not kill you - let alone a yokai. If anything, it could give you mermaid inmortality. Which for fishgirl-chan is a no-issue anyways :-D