I think this video explains pretty well what's wrong with Mario's hair, especially when it goes into further detail about this in a 24 minute video.
This is really close to GameXplain tiered video quality about 1 specific thing.
So, in short:
- Mario's hair is a medical anomaly.
- Dark brown mustache and black hair is usually seen in men, but not the other way around.
- If we use Adobe Photoshop and reverse the colors on Mario, then it still looks... "weird".
- Creator of video seeked out a professional medical subject matter expert in this subject, and has quite a lot to say (but it's hard for me to understand).
- The latter part explains and reveals the design of Mario is the way it is, and how it is immensely popular.