Is saying, "This world is not good," considered reviling time?

Cursing time (ad-dahr) is forbidden because its perpetrator is in one of two situations: either polytheism (shirk) with Allah by believing that time is an actor alongside Allah, or cursing Allah if he believes that Allah alone is the Actor.

As for the phrase "ad-dunya mish hilwa" (life is not good), if it is said out of discontent and impatience with Allah's decrees, then it is prohibited. However, if the intention behind it is to belittle the importance of this world in comparison to Paradise, or to dispraise it to show its insignificance, then there is no harm in that.