Laysa is a defective negative verb. In bi-fāḥish, the bāʾ can be understood as an emphatic extra preposition; the word is outwardly genitive while functioning as the predicate of laysa in sense.
Fāḥish fundamentally denotes excess or impropriety. In the context of a neck it means disproportionate, ungainly, or excessively long, not moral indecency. Semantic interpretation must follow the local context.
The negation prevents a potential misunderstanding of the simile: an elevated, graceful neck is not to be imagined as unnaturally elongated.