I think that "It is not good either" is grammatically correct and "also" is used for positive sentences but we can hear sentences like "It is alsonot good", but it is odd and wrong, isn't it? Thank you.
Someone might write in a review of a play "The play is not short. It is also not good." The short, parallel sentences are good style, and both grammar and idiom permit.
It is not at all wrong, but it is also not at all a natural way to say "It is also bad."
Here's a E.g."I'm aware of the rough time she is going through, but it is also not good for her to treat people that way. It's inexcusable!" That is a good example of when it sound's more natural. There is nothing word with the word itself, grammar wise, but it is pretty tense sounding when used otherwise.