Firearms are all about the transfer of energy. Chemical energy produced from burning gunpowder is turned into velocity, projecting the bullet forward. When the bullet impacts, a greater or lesser fraction of that energy is transferred to it's target. More energy dump is better when we're taking about stopping power, as it's this energy that destroys tissue and causes wounds. Hollowpoints, and expanding rounds in general, are more efficient at this energy transfer thing then jacketed bullets, which cover a soft core with a hard coating. The jacketed bullet is more accurate and can achive a higher velocity, but it's construction is not conducive to an effecent energy dump.
So higher muzzle velocity is a good thing. But it's only a piece of the puzzle. A heavier bullet can inherently transfer more energy then a lighter one, and a wider projectile can physically displace more of the target's material. It's been shown that bigger, heavier bullets destroy more tissue on impact, but smaller, faster ones penetrate more deeply into the target. I'm not even going to touch on hydrostatic shock, or shot placement.
I could go on, but there's literally libraries full of this subject, and the literature is redily available. Short form; the .223 round, in full jacketed format, fired by the AR15, is considered by many to be anemic for a rifle round, with lackluster performance. A softpoint, hollowpoint, or other frangible bullet does a better job in this caliber, but one is still constrained by light weight and small diameter.