The Not Mouth says “no”. What else would you expect from something named the “Not Mouth”? The Not Mouth will always say “no”, regardless of the question.
You ask the Not Mouth if it is afraid of heat (ひ). The Not Mouth says, “no”. So you set the Not Mouth on fire. The Not Mouth then screams, “NONONONONONONONONONOOOOOO~!” Looks like someone can’t handle the heat.