In a novel in which so many bad things happen, it's not all that surprising that fear comes to the forefront. It's hard to think of even one character who doesn't demonstrate fear at some point. The thing to keep in mind about fear in The God of Small Things is that it isn't just a reaction to something scary; it's a powerful motivator that pushes characters to act in particular, often dangerous, ways.
Estha's fear of the Orangedrink Lemondrink Man and Rahel's fear that Ammu doesn't love her anymore provoke the twins to run away across the river. Baby Kochamma and Mammachi's fear of social disgrace push them to lock Ammu away and send the police after Velutha. Fear is a mechanism behind many of the major, life-changing moments of the novel, and the result is often more terrifying than the thing that was originally feared.
In The God of Small Things, fear causes people to act in ways that lead to bad things happening.
In The God of Small Things, fear is a natural response to bad things that are inevitable.