My first guess was that it might have been La belle poule" as in the "hot chick" meaning (poule or poulette in French can take this meaning in slang, although I'm not sure how old this slang is - might be too modern), but apparently there is a legend that it went from "La Belle Paule", Paule being the first name of a lady during Francois 1er reign, on to Bella Paoulla in Occitan (regional language from the South of France) to finally make it back as Belle Poule in French again for the first Frigate known under this name. It seems quite convoluted and not supported by real evidence - so in short: it looks like no one knows why, and it does indeed sounds like a funny name to my French ears. Certainly not the sort of name designed to strike fear in the enemy (watch out for the nice chicken!), unless you're a Monkey Island pirate I guess -
https://www.youtube.com/watch?v=E28WrhpTzQA