I'm not able to understand what is the meaning of mortality rate as referred in episode 84 when it is said that universe 7 has low mortality rate so what they meant with that and why they considered destroying universes for that.
According to Dragon Ball Wikia - Mortal Level
Mortal Level (人間 レベル, Ningen Reberu, lit. "mortal level") is the score that the Zeno and Future Zeno use to rank the twelve universes. It is a measuring of the average development, or "quality", of the mortals existing in a universe. The Mortal Level of a universe is directly related to the effectiveness of the jobs of its Supreme Kais and Gods of Destruction (allowing and promoting the creation and development of adequate civilizations and elimination of inadequate ones).
As quoted in Pablo's answer, the Mortal Level is a measurement of the quality of the mortals used by the Kings of All. We're not really told exactly what goes into deciding this value, other than a singular data point from the manga (which did not happen in the anime, however, to my recollection):
In the manga Universe 7 gets enough of a score boost to rank-up (a single position) thanks to 17's wish to bring back all of the destroyed universes. So setting aside selfish desires to save things beyond yourself that normally would never affect you is apparently one of the dimensions for evaluation.
We're also not explicitly told why this measure was developed, etc. But the implications are clear: the universes were created for a purpose, and that purpose is not simply to exist and unfold in whatever way they desire. The creator god(s) are not simply watchmakers and observers, but are actively engaged in their creation for whatever that purpose is. That purpose is evidently measured, at least in part, by Mortal Level. Universes which systematically fail to live up to that purpose are subject to destruction. Perhaps the Kings of All had a motive similar to the creator gods in the Tenchi series: to see if they were powerful enough to create something more powerful than themselves. If so, maybe that's why Whis was always hoping Goku and/or Vegeta could achieve Ultra Instinct. Perhaps they just wanted companions and entertainment the angels couldn't provide, and created universes so that they might produce such beings, and universes that seem incapable of doing so can be discarded. Perhaps it is all just a form of entertainment, as we have at times seen the Kings of All acting bored and wanting something interesting to happen. In which case the Mortal Level might just be an arbitrary measuring stick created simply to give a sense of challenge and excitement: see how high the universes can go, see what you can do to get a higher score, but with no purpose beyond the score. Universes that do poorly could then be discarded, like one might delete or reset a video game save that doesn't go as well as one desired. Got caught pickpocketing in Skyrim? Reload and try again!
One way or another the Kings of All are meant to be alien: their values and goals are fundamentally different from our own, or from (most of) those inhabiting the universes.