>>52581853Many notable trainers wear fashion/accessories that relate to a particular Pokémon that fits their theme, even if they don't have that Pokémon on their team. The most obvious recent example is Iono and her literal magnemite accessories, though there are some subtler instances too.
Following this line of logic, the key is probably a reference to Klefki:
>Steel/Fairy type (fairy is the "cute" type)>Kalos Pokémon and the lead of the Kalos Steel-Type Elite Four member Wikstrom (whom Poppy may be related to, given her anachronistic outfit and occasional archaic vocabulary)>The key is on a necklace specifically (Klefki is a key ring)>Toy keys are common toys for toddlers; Poppy is not a toddler, but she is still young enough that the imagery can be reasonably applied 
