Is it OK to change mistakes/typos when you are in the minor revision stage of publication?
Yes, this is completely OK. You are not bound to perpetuate typos from earlier versions just because reviewers did not find them.

It's always good to indicate that typos were cleaned up and similar, though you shouldn't note every single typo. I'd write something like "we corrected typos, especially in section X, and tightened up the prose in paragraph Y" in the answer letter - just to make sure that nobody thinks you are trying to smuggle in larger changes.

Of course, there is a bit of a gray area between small and not-so-small changes. If you want to change something that goes beyond simple spelling errors or awkward prose and starts to change your argument, you should draw the editor's and reviewers' attention to this more explicitly, since they should really sign off on any not completely minor changes at this stage.