I was surprised myself at how good I nailed the interview.
So here the speaker is trying to convey that they were surprised at how well the interview went and how well they, the speaker, performed. But would this sentence be considered grammatically correct?
While good is often used in the sense of well, as in:
We are all good; thanks for asking.
It doesn't work in your sentence; you need to use well - and the use of myself, while legitimate for emphasis, is unnecessary.
Moreover, to nail something in the sense of achieving/accomplishing it is slang. So it's fine in certain contexts and dubious in others. In short, you might rephrase as:
I was surprised (myself) at how well I nailed the interview
or, more formally:
I was surprised (myself) at how well I did/succeeded in the interview.
https://www.urbandictionary.com/define.php?term=Nailed%20it
https://dictionary.cambridge.org/dictionary/english/nailed
