"My grandmother has bad hearing."
んです and its other forms (such as んだ、のだ、ので) are used to indicate an explanation. Tofugu explains it pretty well at https://www.tofugu.com/japanese-grammar/explanatory-nda-ndesu-noda-nodesu/. For example, it's Sports Day and your kids are supposed to be at school for the big performance with their class. You're at the bus stop with your daughter, but your son is missing, and the teacher looks surprised and disappointed when you say he isn't coming today. You feel her uncomfortable stare and admit -息子のお腹が痛いんです。 -むすこのおなかがいたいんです。 -It's because my son's stomach hurts. Without んです it still says that you're son's stomach hurts, but now it's a random statement not inserted in the rest of the conversation. んです clearly ties it in as an explanation to the previous question (or, in our case, the implied question) Tl;dr it means that you're providing an explanation or extra information
by the way does it make sense if i there's a ん at the end?
It's okay to use it, but it would slightly alter the meaning. んです is more or less untranslatable. But the overall nuance is
- 「悪いです」：it is bad.
- 「悪いんです」：it is getting pretty bad (as when comparing to what it used to be).
No, 悪い is an i-adjective, so it can't be followed by だ.
Read more at Tofugu: https://www.tofugu.com/japanese-grammar/da/#using--with--adjectives-and-verbs