Translation:It is neither good nor bad.
I think this answer is incorrect. The question is what do you want to say with an answer like that? I think that in English there are two possibilities:
1) "I am neither good nor bad (as a person)." However, in Japanese you do not say that someone is simply いい, you have to say いい人.
2) "I am neither good nor bad (at something)." In Japanese you would say 上手 and 下手.
I think that makes sense. If I may attempt to break it down:
よくもないです。"It also isn't good," or maybe "it isn't good, either."
わるくもないです。It isn't bad, either.
On their own, these seem to be referring to some other quality "it" was stated to have earlier. Like "well, it isn't spicy," and you respond "it also isn't good!"
So when you say them in the same sentence, they play off each other, literally becoming "it also isn't good and it also isn't bad." Without that second も it definitely is ambiguous. It might just be an incomplete sentence, since the わるく isn't part of the も list and yet よく is sharing its ない.
That's the other thing this sentence is teaching us, I think: く-form adjectives aren't always followed immediately by ない. In lists, as this example shows, and probably other times as well.
I think the nature of も requires that the other thing also be negative, if this one is negative, or positive if this one is positive; they must match. So you probably wouldn't say 「つらくないです。おいしいもです。」