Is one who stumbles upon something inscribed with "Makkah Al-Mukarramah" in a dark room, due to his laziness and failure to turn on the light, considered sinful?

You have done well by respecting something upon which something venerable is written, for respecting what is written upon something revered in religion is a part of exalting the حرمات (sacred things) of Allah and His شعائر (rituals/symbols). And the name of Makkah is among what should be exalted. There is no blame upon you for what happened, because you did not intend it. Allah Almighty said: "And there is no blame upon you for that in which you have erred, but [there is blame for] what your hearts intended. And ever is Allah Forgiving and Merciful."