I feel like Wakasa being secretly smart isn't that much of a stretch. All throughout this manga we've seen how curious and inquisitive she is, usually about mermaids or their tails. And it's not entirely perverse on her part, either, it's genuine love.
... though, I also want to point out as someone who was a special education students, good grades doesn't necessarily indicate intelligence. Smart people can test really badly or vice versa. I'll spare everyone the rant about the Prussian-style of education system because it's got nothing to do with cute mermaids.
Eagerly anticipating the final chapter, even though it means saying goodbye.