It doesn't make sense to me either. Maybe the point is that if someone hits you with a brick, it doesn't matter whether you have nut allergies or not. Not a very good joke, in my opinion.

I have a joke in my native language that means the same thing. It probably means that there are so many people with nut allergies that if you throw a brick at a crowd, you will hit a nut allergy.

Sometimes memes like this hardly qualify as a joke and it's very difficult to explain...but here's my attempt: People on the internet like to complain about everything, so in this case, I believe the purpose of the meme is to diminish or disrespect people who have or claim to have a nut allergy. Some people either don't believe people have nut allergies or they just think that the sudden, dramatic rise in nut allergies (or the people who have/claim to have them) is a sign of societal weakness.