One thing is smooth-shading - it can look bad, but it can also look just fin if you do it right. For a more "real" fix, assuming the faces are directly along one axis (in your case, Y), you can select the faces that are supposed to be flat (in your case, the faces that are triangulated), and scale to 0 along the normal axis (in your case, the X axis).
If they're not along a specific axis, go from Global transform orientation to Normal. That way, when you select the faces, click the Scale tool, hold Control, and scale to zero.