Do lower back muscles help flatten stomach muscles?

Not strictly. You can't actually do much to flatten abdominal muscles, but you can 'flatten your belly/tummy' by losing the fat that covers your muscles. This answer will take it from that perspective.

All muscle groups operate in pairs, typically called flexors and extensors. Your biceps flex (bend) your arm at the elbow, and your triceps extend it. Your core is no different; your abdominal muscles flex your trunk and your lower back muscles extend your trunk.

Since muscle groups are paired, you should work both parts of the pair equally (not necessarily at the same time or the same day, but with equal intensity and attention throughout your circuit). Failure to do so can result in muscle group imbalance, and even lead to failures of health and injuries.

Development of your lower back is critical to successful development of your abdominal muscles, and therefore contributes to having a flatter tummy. But the absolute most critical thing you can do to get a flatter tummy is to lose the fat that covers your abs; you already have abs (everyone does unless they've been surgically removed or were in some bizarre accident), you just need to peel away those adipose layers covering them up!