He was never "convicted". He was specifically named in an indictment as an "unindicted co-conspirator", because the prosecutor advised the grand jury that they could not legally indict a sitting president (or maybe he just didn't want to - I don't know who put that phrase into the indictment). After this, he resigned rather than face a probable impeachment. He claimed to be innocent of the charges, but that he resigned to spare the nation the pain and paralysis that an impeachment process would bring.

"I'm not a crook" is memorable because it's pithy and funny. The version I remember hearing (second-hand) is: "I am not a crook, I've never been a crook:.... I don't even know what a crook looks like". This may not be an actual quote, but a lampooning of it. But that's the thing - it's easily lampooned, and easily remembered.

I don't recall this quote being "scrutinized", per se. As an off-hand comment from the president, it has no legal force behind it. It's not like he was under oath. Also, from the context he seems to be claiming that he's never engaged in political acts in order to personally and financially profit. This may well be true, but is a far cry from denying any wrongdoing while in office, which is what people wanted to know about.

So aside from comedic value, it was pretty much ignored.