this will add an elevation effect to your app bar
TopAppBar(modifier = Modifier .padding(10.dp) .shadow( elevation = elevation, spotColor = Color.DarkGray, shape = RoundedCornerShape(10.dp) ))
[![top appbar with elevation][1]][1]

[1]: https://i.stack.imgur.com/D9SOL.png

this works very well for me:
TopAppBar(modifier = Modifier .padding(10.dp) .shadow( elevation = elevation, spotColor = Color.DarkGray, shape = RoundedCornerShape(10.dp) ))
[![top appbar with elevation and rounded corners][1]][1]

[1]: https://i.stack.imgur.com/CDegH.png