Board Thread:Questions and Answers/@comment-39658513-20190601020815/@comment-5398287-20190601130554

At the time that he learned the Saigo no Getsuga Tenshō and by extension Mugetsu, Ichigo had access to his Quincy powers masquerading as his Shinigami powers, a small fragment of his Zanpakutō powers (that allowed him to perform Konsō and the like), and his Hollow power. It's a big part of the final arc that he finally unlocks his true Shikai/Bankai and gains a huge power boost as a result. And as you say, Mugetsu was all of his available power used at once; if he used just his Hollow powers, it's unlikely that he would've beaten Aizen. Put up a good fight and maybe overpowered him, sure, but not so utterly thrashed him that the Hōgyoku would give up and let him be sealed like what happened in the storyline.