Normal Mirage Spots have nothing to do with time.
As far as the regular, day-to-day Mirage Spots go, the time of day does not affect what Pokémon are encountered. Porygon can be found on one random Mirage Island that varies from day to day. To increase your chances of encountering that island, activate the PSS and you will recieve up to four different Mirage Spots a day, as long as you go on the PSS every day. Here is a link about Mirage Spots, and which one has Porygon. Good Luck!