Your first issue is that you're testing to see if the point clicked is exactly the position of the ball. This is highly unlikely, and almost certainly not what you want. Instead, you want to test to see if the point clicked hits anywhere in the ball.
Your second issue is that you need to transform your device coordinates to box2d coordinates. The device and box2d use different coordinate systems, so comparing the two doesn't make sense.
To solve the first issue, you can loop through all of the fixtures in the body and use the method fixture.testPoint(x, y)
to see if the point hits the fixture. Since your body only has one fixture, you can just use this code for the test:
body.getFixtureList().get(0).testPoint(x, y);
Solving the second issue is a bit trickier. Ideally, your game should be using a camera to handle rendering. If it doesn't, you should get one. With the camera, you can unproject the device coordinates to get camera coordinates like this:
Vector3 mouseCoords = new Vector3(screenX, screenY, 0);
camera.unproject(mouseCoords);
//mouseCoords.x and mouseCoords.y will now be relative to the camera
Then, you have to convert to box2d coordinates:
mouseCoords.scl(WORLD_TO_BOX);
where WORLD_TO_BOX is the scaling from world coordinates to box coordinates, which you should already have setup. You can then use mouseCoords.x and mouseCoords.y in the testPoint
method.
EDIT: Code below shows how to loop through all fixtures in a body. This isn't necessary if you only have one fixture, however.
for(Fixture fixture : body.getFixtureList())
{
//Do something to fixture
}