While it is good to always aspire for greatness or betterment, it does not have to lead to faking who or what you're not. Nonetheless, I think faking to be something when you're not depends on the outward look of such thing and how it would affect your relations with people.
Faking it means you already acting like you're it and that's dangerous because people begin to see you in that light whereas, you're not there yet which when people get to find out that you ain't there yet, they won't look at the fact that you were perhaps faking it as a means of encouraging yourself that you can do it, they'd see you as a fake person who claims to be what he is not.
Personally, I wouldn't advise faking it until you make it cos, it's more like lying, pretending to be who you're not. I'd however suggest that, faking it should be a thing of the mindset and not necessarily outward disposition.
Tell yourself you're a great person, you're rich, you're this and all you want to be, as a means of encouraging yourself. That in my own opinion is the best way to "fake it until you make it".