Let's not beat around the bush here: we just don't like Baby Kochamma. She probably wouldn't like us either. Baby Kochamma is endlessly preoccupied with impressing the "right" people. She's so concerned about not looking bad in front of others that she's willing to undermine anyone and everyone. Her favorite people to pick on are Estha, Rahel, and Velutha. When she realizes that Estha is quietly laughing at her for being pompous in front of Margaret Kochamma and Sophie Mol, she makes fun of his Elvis Presley getup. She takes great pleasure in making sure that nobody else is happy if she can't be happy.
What also makes Baby Kochamma the major antagonist of the novel is her willingness to throw people under the bus (metaphorically) when her own butt is on the line. The most striking example is when she gets Estha to lie to Inspector Thomas Mathew by making up a story about Velutha kidnapping him and Rahel. Baby Kochamma is a miserable, selfish woman.