one of the users of mad doctor, from one piece, is on the tv tropes page of mad scientist but on his character page on one piece, it doesn't say he is a mad scientist more of a mad doctor. just something i wanted to tell you.
Those categories both got nuked because people added to them without considering if it made sense. So we ended up with shit like "Apocalyptic Artillery" in Non-Fighting Powers. Please do not add more powers to Non-Fighting or Fighting Powers.
The reason its not a power is because using your belly to attack is more of a technique then an actual method of combat in of itself.
It falls under rules 1 and 2 of the new power requirements-
1) Must not be readily or easily accessible/attainable by a normal human.
2) Must not be an ability, function, or condition already present within a normal human populace.
Based on the evidence it doesn't fit the necessary validity to a be a power on the wikia.
As such it falls under the Not a Power category by most basic definition and ruling.
Anyone can use their belly to fight with, both humans and animals alike.
The other two powers you cited as being examples of why the page should belong are actually far more difficult to achieve in reality, and not just anyone can actually pull them off like its shown in fiction.
If you tried to fight using breasts, your more than likely to injure yourself. And fighting with your hips and butt wouldn't work without knocking yourself off balance or causing pain in your spine and back.
So there is a very good reason WHY its not considered a power. Its really nothing more than a basic technique of someone with a large body weight.