"Ona jest dobrą babcią."

Translation: She is a good grandmother.

[Jack.Elliot]

she is a good granny

reported so many times

[CelineKmoc]

Why "dobrą"?

[Jellei]

"babcia" is feminine, 'good grandma' is a noun phrase, so in such a sentence it has to take Instrumental, which is "dobrą babcią".