If a person passes away on Shabbat, does that mean they are righteous?

It all depends solely on the deeds of the individual in this world.

It is indeed stated in various texts that a person who passes away on Shabbat or a holiday, as well as on a Friday afternoon after midday, are times when the attribute of mercy is dominant. Dying during such times is a reflection of kindness and an expression of mercy, which indicates a level of the deceased and their association with the good days and the World to Come.

However, one should not conclude that if a completely wicked person, Heaven forbid, passes away during these times, it would be determined retroactively that they were righteous. There are, of course, those who pass away during these times due to the attribute of justice, which demands retribution from the wicked, and this is also an act of kindness for the world.