First of all, the movie had to live up to the wonderful Roz Russell version. So it had to have a huge star to compete.
The story has it that Lucy broke her leg while skiing so that may have affected her dancing ability. She was known for physical comedy, so it's not like she couldn't make her body do the dancing.
But the fact remains that Lucy was never a good singer. She could do comedy numbers, but she never under any circumstance in her entire career could pull off a ballad like "If He Walked Into My Life". She should have swallowed her pride and allowed her music to be dubbed or sweetened.
She should have realized that her Broadway musical career (Wildcat - 12 years before) was not that successful and just admitted to the fact that she's really not a musical comedy star.
Additionally, what makes the movie so bad is the tricks they used to film her. That movie has more soft focus moments than any filmed dream sequence.
If anyone ever tells you that you put too much Parmesan cheese on your pasta, stop talking to them. You don't need that kind of negativity in your life.