In response. No.
Sleeping in a fetal position compresses and cramps the muscles and the abdomin.
To tighten abs, one must exercise the ab muscles by doing situps, weight lifting, or using one of those ab shaking vibrating belts advertized on TV, or one can practice resistance training exercises.
In Chinese martial arts, there are ab exercises that involve the horse stance (ma bu), deep breathing, breath-holding, and inner energy circulation (yun qi) exercises with arm and hand movement.
One such inner energy circulation (yun qi) exercise is to sit in the lotus position, place the hands on the thighs with palms facing up, inhale as one raises the hands and arms up to the neck as if pulling an imaginery heavy weight, then turn the palms down, and push downward as if pushing something down while exhaling.
Another such exercise is to stand with feet apart, then imagine using both hands and arms to pull up a very heavy imaginery wooden bucket of water up to the waist, then holding that imaginery bucket of water while slowly lowering it.
Both of these exercises are resistance training exercises that specifically target the ab muscles.