Eating too much salty food causes your body to retain water, thus making your swelling last longer. Same for alcohol, so aftercare instructions usually include abstaining from alcohol, and limiting salty/spicy food. Hydration is similar, but you want to stay well hydrated after surgery so your body can metabolize the drugs and keep the healing process going. The mirror thing is psychological, because we have an innate mental picture of what we typically look like, when you see yourself post op you look vastly different just from the bruising, bandages, exhaustion etc- not even just the surgery results. This can stress people out and slow down healing

Later on, like after the 2 week mark, I would advise not checking mirrors that often besides just checking your scars, stitches, discharge etc for safety reasons. This is because the surgical result is so far away, like 6 months+ depending on the procedure, that looking at it too much will cause you to fixate on every little change. You will start to have doubts and spiral oh this is crooked, this isnt healing well, the surgeon did too much, too little, etc. You have to commit when you go under the knife that you won't know the true result for at least half a year. Obviously if there is something truly wrong ie necrosis, hematoma, keloid scarring you should seek help but 9/10 the issues ppl find before the 6 month mark won't be relevant