Would there have been any danger to the astronauts or the equipment if a soft item, like a crumpled up napkin or tissue, was loose during the Apollo CM reentry, or did everything have to be secured?
Following suggestions in the comments, below is an attempt of the answer, although it is based rather on a common sense / engineering guesswork.
There was an incident during Apollo 12 splashdown, but with a hard item:
During splashdown, a 16 mm filmcamera dislodged from storage and struck Bean in the forehead, rendering him briefly unconscious. He suffered a mild concussion and needed six stitches.
This is the real life proof that things can get "flying" and impacting if unsecured during reentry/splashdown.
- If the item in question is soft and light (akin to suggested napkin) there might be a chance of potentially clogging suction vent for air recirculation;
- Although even less likely, if the item is soft but heavy (whatever the item might be: here are few examples from one of the real Apollo stowage lists: a food/hygiene item or liquid cooling garment 4.6 lb, etc.) when unsecured and subjected to moderate-to-high g-forces, can potentially actuate an odd switch or circuit breaker by impacting on it.
I wouldn't expect severe consequences of a soft item impacting directly on human. Soft and light objects would obviously do no harm at all, and soft but heavy objects would give a feeling similar to a moderate hit by a pillow or very light hit by a soccer ball, i.e. near zero likelihood of life threatening.