It's not unusual for enemies in games to have weird weaknesses; hell, some games have bosses that are completely immune to basically everything but an attack to their weak point or from some obscure element. That said, minotaurs in most stories are typically on the more unintelligent side of things and are thus incredibly weak to mental spells, which a paralyzation spell would definitely be (if you wanted it to be efficient, anyway; you could have it lock up the muscles, but it's far simpler to just prevent any signals to the limbs from leaving the brain).
As for why the goddess deemed him worthless despite him being a solid debuffer, it's probably because she's an incredibly vain piece of shit and debuffs don't fit her "heroic aesthetic". She probably wants the knight in shining armor, the majestic wizard, or the dashing rogue; not someone who, from her point of view, just sits on the sidelines contributing little.