This was four months ago so you're def way past this, but -지 않다 makes the statement mean "not 많다" or "not a lot", so it this sentence doesn't mean the same as the one you proposed. It's like when it snows and you have a convo with a friend that goes: "Is there not a lot of snow by you? Wow, we got 10 inches".
I don't think this is correct. I might say this to someone if we were both looking at something and I thought it was a lot of frost, but he or she thought it was dew or something other than frost, or called it as such. "That's a lot of dew," my friend says about what I thought is frost. My reply: "Is it not a lot of frost?" Or perhaps the weather report predicted a lot of frost, but my friend talked about the weather report as if it had predicted snow. I might say about the prediction, "Wasn't it a lot of frost? Now snow?"
The accepted translation, or the Korean sentence, is something I would say if the other person and I both agree it's frost (or at least the listener hasn't said anything to indicate otherwise), but may or may not agree about how much it is, or whether that amount is a lot.
The difference is that the statement of disagreement with "Is it not a lot of frost" or "Isn't it a lot of frost", that is, if you say, "no, it's not," that could mean denying the entire claim about the subject "it"--that what ever "it" is, it's not "a lot of frost." Whereas, to disagree with, "Is there not a lot of frost," that is, to say, "No, there isn't", the claim you're disagreeing with is that the subject "frost" exists in large quantity ("there is", "is there" basically mean exists, and "there is a lot of" means). You're not disagreeing with what something is, but with how much something is. Same for the Korean sentence. The frost 서리 is the subject, and the predicate is essentially asking whether there is a lot of it.