I don’t know the origin of the phrase It’s not what it’s cracked up to be, and I’m not particularly interested in finding out. The phrase is overused and doesn’t make sense to most people who use it.
It’s not what it’s cracked up to be is often used as a way of explaining that something isn’t as good as was advertised or expected. That terminology must have made sense at some point in our history, but few people saying it now know that history. They’re just spitting out a long-outdated phrase that has no literal meaning in our current language.
I know that there is a segment of our audience who doesn’t care about whether a phrase makes literal sense (or about following traditional grammar rules, for that matter). There is a larger segment, however, who is interested in questioning the meaning of what they say in an attempt to choose more articulate, effective language (and learning traditional grammar rules).
All I know is that this phrase isn’t what it’s cracked up to be, whatever that means.