Does stomach muscle tightening exercises help in reducing symptoms of mild L5 spondylolisthesis?

Yes. No treatment works 100% of the time, but strengthening the back and tummy (abdominal) muscles is helpful for many people with back problems.
Definitely. This is an excellent way to address spondylolisthesis. Core stabilization and strengthening along with pelvic floor and low back strengthening together are an excellent approach.