Kid is attacking yonko shanks fleet because he is a pirate that wants to conquer and win, what does it matter if they are strong or weak? You cant just attack one person you have to attack the whole army. Thats what real life conquerors like genghis khan had to do. No one in history has conquered anything being a mickey mouse.
Kid doesn't even know they are weak. Its just oda saying they are weak to potray shanks as the hero that protects the weak, so that all the kids and manchildren watching can be like "oh hes sooo coool"