This question proposes a hypothetical birth defect. If you accept the proposal then the only answer is yes: hypothetically the resulting "insane amount of muscle mass" could stop a bullet. Bullet design is a precise science and manufacturers calculate the stopping power of bullets by estimating tensile strength of muscle (roughly 1 to 4 MPa (145 to 580 lbf/in²) according to wikipedia). If the tensile strength of your muscle is increased, whether by working out, bio-engineering or random mutation, then a larger, heavier, faster bullet will be required to penetrate it.
Superhero stories can belong to either the sci-fi or the fantasy genre. The difference can be illustrated by comparing early Superman stories (where his fairly reasonable super-strength is given a far-fetched scientific explanation) with the planet lifting, x-ray sighted, ice-breathing, light-speed travelling Superman of later years, who clearly is a fantasy figure. But whether the powers in question are given a scientific or fantastic context, an examination of the physics of such superpowers would usually reveal that either the practise of the powers or their explanations contradict the laws of physics.
For instance, Superman's strength is caused by coming from Krypton, a planet with a much higher gravity than Earth. This might possibly account for the earliest Superman feats where he lifts cars, but could he lift a planet like Earth (which he has done) weighing 5,980,000,000,000,000,000,000 metric tons? Very generally, if you assume a healthy young guy like Kal-el (Superman's name on Krypton) can lift his own weight, he would have to weigh 5,980,000,000,000,000,000,000 metric tons on Krypton. This would mean that Krypton would be a star of greater mass than our sun, with nuclear fusion occurring as a result of the massive gravity.
Essentially, even if you can accept the possibility of such hypothetical mutations, you cannot hypothetically suspend the laws of physics. Superman would have to eat an impossible amount of carbs to create enough energy to achieve such a feat as lifting an ocean liner. In the context of the original question, being shot while wearing a bulletproof vest may not result in being penetrated by the bullet, but as the force of the impact blows you through a shop window into the path of a bus, the final thought that runs through your brain might be that you wish you had paid attention in physics class the day they discussed Newton's laws of motion.
Even in the world of comic books physical superpowers are somewhat limited. Super-villains realise that defeating superheroes is as easy as creating access to more and more powerful weapons. Look at Batman - a non-mutated regular human who achieves superhuman crime- busting results using superior yet plausible technology and intelligence. The latest Bat-shield must be the equal of the Joker's latest weapon - a typical characterisation of an East vs West arms race which is a mainstay of comic books. This prompts the thought that the hypothetical physical advantage proposed in the original question is irrelevant when you consider that the real life arms race on Earth has already produced weapons that could easily destroy every superhero in the sci-fi canon. Besides, the most important life preserving characteristic that Batman possesses is the most regular thing about him - his anonymity.
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