Is there any effective way to generate custom shape in flutter?

You can do this with CustomClipper like this:

class CustomDraw2 extends CustomClipper<Path> {
  @override
  Path getClip(Size size) {
    var path = Path();
    path.moveTo(0, 0);
    path.cubicTo(
        0, 0, size.height * 0.3, 0, size.height * 0.4, size.height * 0.4);
    path.cubicTo(
      size.height * 0.4,
      size.height * 0.4,
      size.height * 0.4,
      size.height * 0.66,
      size.height * 0.9,
      size.height * 0.7,
    );
    path.cubicTo(
      size.height * 0.9,
      size.height * 0.7,
      size.height * 1.4,
      size.height * 0.7,
      size.height * 1.5,
      size.height,
    );

    path.lineTo(0, size.height);
    path.lineTo(0, 0);

    path.close();
    return path;
  }

  @override
  bool shouldReclip(CustomClipper<Path> oldClipper) {
    return false;
  }
}

and use it like this:

Container(
    clipBehavior: Clip.antiAlias,
    margin: EdgeInsets.all(16),
    decoration: BoxDecoration(borderRadius: BorderRadius.circular(8)),
    child: Stack(
      children: [
        Container(
          height: 100,
          width: double.infinity,
          color: Colors.green,
        ),
        SizedBox(
          height: 100,
          width: 300,
          child: ClipPath(
            clipper: CustomDraw2(),
            child: Container(
              color: Colors.blueAccent,
            ),
          ),
        ),
      ],
    ),
  )