There's really no rule for wall thickness. You can make the walls thick like you did or extremely thin, in the 1/16 inch range and light will come through.
The oval shape is because the wood wasn't dry when you turned it and as it dried it warped a bit. If you don't want it to warp you need to either start with dry wood or you can rough turn the wet bowl leaving the walls thick, let it sit for some time (months) and then remount it on the lathe and finish turning it.
BTW, some bowl turners deliberately let the bowls warp. You can even use clamps to shape the warping.