Hey, I'm not stuck on this one specifically but I'm also stuck on a Kanto one.
The poem goes:
Love is like a rock thrown,
You can't see it coming but the feeling,
Unswerving like Marowak's love for Cubone.
Any help is much appreciated.
For yours I woulda thought the poem pointed towards route 17/16. Maybe try to interact with every NPC there, maybe you missed one.