What is the ruling on speaking about the worldly life as if it were treacherous, unjust, or cruel, such as saying: "If there was mercy in you, O worldly life, then where is the sign? What happened to you, O worldly life? Why do you resist me? What was my only mistake? And why does my heart suffer?"?

The worldly life (dunya) is neither to be condemned nor praised absolutely. However, its condemnation is more prevalent in texts because most people indulge in its desires. The statement mentioned in the question is not a dispraise of the worldly life in a religious sense, but rather a complaint and rebuke against divine decree. Objecting to divine decrees is forbidden. A Muslim must be patient with Allah's judgment and know that what Allah has decreed is good for him, and that any calamities that befall him are due to his sins. Therefore, he must repent and supplicate for the betterment of his condition.