她是好妈妈
Because 妈妈 is a noun.... what are you confused about? 我是好 is grammatically incorrect, 我很好 is correct.
我不是很好
This is fine, because @DodecaWeasel brought up the context, that being 我也在学习说普通话. You said "you don’t use 是 with adjectives" without giving any exceptions.
The other poster seemed to have understood what I meant, I think it was clear. You simply don't use 是 with adjectives in Chinese the way you would use "to be" with adjectives in English. In your supposed counterexample the 是 is for the noun, 她是妈妈 is grammatical, 她是好 is not. 她很好 is correct.
I know the other poster understood you, I'm trying to say that the other poster might assume you're right in every situation when that's just not the case. Since they provided the context, that being they're studying Chinese, when they say 不是很好 (I'm not very good), it's correct because the context serves as a noun of sorts (referring to their standard). You try saying stuff like 我很不好 (I'm very not good), you'd be laughed at in a conversation. Other exceptions like the question: 我是不是很胖? (Am I or am I not fat?) is another example we can use.
seems like the way to do it for whatever language you're trying to work on. Also try changing your browser/PC/phone locale when you're ready just to change things up a bit more.
