Is birthday Pikachu legal?

Inquiring minds want to know: Is the concept of a "birthday Pikachu" a legitimate one, or is it merely a novelty item that exists in the realm of fan culture? The question begs to be answered in the context of both legal frameworks and cultural norms. Does the creation and distribution of such a character variant abide by copyright and trademark regulations? Is there any precedent for the legal recognition of such personalized versions of copyrighted characters? Furthermore, does society at large consider such a personalized interpretation of a popular character to be acceptable, or does it violate some unseen norm of fan engagement? The legal and cultural implications of a "birthday Pikachu" are ripe for exploration.