「悪{わる}く(は)ない。」, contrary to what you seem to believe, does not really mean "It is not bad.". At least, that is not what the phrase means all the time.
"It is not bad." is only the dictionary or machine translation of 「悪{わる}く(は)ない。」. It fails to include the nuance of the Japanese phrase.
When native speakers such as myself say 「悪{わる}く(は)ない。」, they often imply that the object of the discussion is not too good. 「悪{わる}く(は)ない。」, in Japanese, would suggest that unstated negative connotation more often and/or strongly than the phrase "it is not bad." does in English.
This is why I, as an English-learner, was quite shocked some years ago when I found out that English-speakers seemed to often say "not bad" in situations where they clearly meant to say "pretty good". I am not saying that this usage of 「悪{わる}く(は)ない。」 never occurs in Japanese, but I can assure you that it is rarer in Japanese.
Thus, depending on the situation, the speaker, the intonation, the phrase 「悪{わる}く(は)ない。」 can sound condescending, negatively judgmental, etc. (That is called 「上から」 in colloquial Japanese.)
Thus, a safer phrase choice for Japanese-learners would be:
・「(なかなか)いいですね。」
・「いいと思{おも}います。」 , etc.