Warning: spoilers ahead for My Hero Academia chapter 332!
Top American hero Star and Stripe would have been able to definitively prove that she's stronger than All Might if she'd defeated All For One, but as of the latest My Hero Academia chapter, she seems to have lost. And yet, a flashback gives the impression that she is indeed tougher than her spiritual master.
Recent chapters have demonstrated beyond a reasonable doubt that Star and Stripe's New Order quirk is absolutely terrifying. She successfully suffocated All For One by removing the air around him, grabbed laser beams out of the sky to form a massive sword, and created a giant version of herself 1,000 times her size out of the atmosphere. She would have also been able to stop All For One's heart from beating if she hadn't wrongly thought he was Tomura Shigaraki. And yet, incredibly, her quirk was apparently useless once upon a time.
In chapter 332, a flashback shows Star and Stripe being taunted for having an incredible quirk but not being strong enough to make good use of it. New Order allows Star and Stripe to change a rule for herself or whatever she touches so long as she meets certain criteria while announcing her decree (like being able to name her target accurately.) When learning how to control her quirk, she tried both strengthening her body so she could fully handle the alterations to reality she makes and coming up with different orders that weren't as extreme. But, apparently, the results didn't impress her or her colleagues. This is hard to believe after witnessing the miracles she has performed in the modern day. Even All For One seems to covet the New Order quirk more than One For All. He's referred to obtaining the latter as just the midgame goal, while saying that acquiring the former would make his war just a formality.
In other words, Star and Stripe molded New Order into such an impressive quirk herself, and in the world of My Hero Academia, those who work hard undoubtedly have more of an edge to them than those who arrive at the same destination effortlessly. All Might, in comparison, was lucky enough to inherit One For All. Yes, he was naturally gifted to the point where he could harness 100% of it without hurting himself when Deku couldn't. But even when Deku could only wield just a mere percentage of that full power, he could still deal an incredible amount of damage that amazed spectators and terrified opponents, just at the cost of terrible injury.
In other words, neither Deku nor All Might had to alter or improve the quirk. Star and Stripe seemingly did by transforming an ability that didn't impress anybody into what is easily the most ridiculous quirk in the entire franchise. What readers are undoubtedly waiting to see is how her quirk manifested originally and what she did to get where she is now. Did she somehow strengthen her body to an absurd degree? Whatever she did, it put her on equal footing with All Might during his prime, and showed that her power is the result of her own blood, sweat, and tears in a way his isn't.
But while Star and Stripe may actually be even tougher than All Might, she'll never be anywhere near the hero - My Hero Academia fans have already discovered she's a horrible person, and if Deku's journey has proved one thing, it's that a hero is defined by how they connect with and protect others, not what they'll do for power.