I'm not a licensed embalmer, but I work as an assistant at a funeral home and have seen a lot of things. There are several variables involved with a deceased person being presentable for viewing. In Washington, and other but not all states, embalming is required for a public viewing, but a limited private viewing without embalming can be arranged for family members, usually a maximum of one hour and ten people. The funeral home will comb the hair, apply light makeup, set the features (eyes and mouth closed, hands folded over the body) and dress the remains in clothing brought by the family, or in a clean hospital gown. Our funeral home also has a room where the deceased can be viewed through a glass wall, in case odor is an issue.
Several factors influence the success of embalming for short term preservation and a good appearance. Typically the sooner the embalming can be done after death the better. Several times when I've brought someone in during the night or on a weekend / holiday one of our embalmers was waiting to start if they knew the family wanted a viewing. An autopsy, physical trauma to the body (such as an accident), or someone with poor circulation present special challenges, though I've seen some amazing results. The skill and experience of the embalmer obviously plays a roll in the outcome with difficult cases.
Last June I went to the visitation for someone who died on May 16 at age 95 but the viewing wasn't until June 17. I had not seen her for 7 or 8 years, but she looked just as I remembered her. On the other hand, an uncle who died of natural causes at age 95 and whose Rosary and viewing were 4 or 5 days later looked like a store manikin (this was at a different funeral home).
Funeral Director's will advise families to not have a viewing if circuмstances suggest that would be best, but often
they need to follow the wishes of the family.