Supreme Court talks as follows regarding Mental Cruelty:

The question that requires to be answered first is as to whether the averments, accusations and character assassination of the wife by the appellant husband in the written statement constitutes mental cruelty for sustaining the claim for divorce under Section 13(1) (i-a) of the Act. The position of law in this regard has come to be well settled and declared that leveling disgusting accusations of unchastity and indecent familiarity with a person outside wedlock and allegations of extra marital relationship is a grave assault on the character, honour, reputation, status as well as the health of the wife. Such aspersions of perfidiousness attributed to the wife, viewed in the context of an educated Indian wife and judged by Indian conditions and standards would amount to worst form of insult and cruelty, sufficient by itself to substantiate cruelty in law, warranting the claim of the wife being allowed. That such allegations made in the written statement or suggested in the course of examination and by way of cross-examination satisfy the requirement of law has also come to be firmly laid down by this Court. On going through the relevant portions of such allegations, we find that no exception could be taken to the findings recorded by the Family Court as well as the High Court. We find that they are of such quality, magnitude and consequence as to cause mental pain, agony and suffering amounting to the reformulated concept of cruelty in matrimonial law causing profound and lasting disruption and driving the wife to feel deeply hurt and reasonably apprehend that it would be dangerous for her to live with a husband who was taunting her like that and rendered the maintenance of matrimonial home impossible.

Precisely,

As to what constitute the required mental cruelty for purposes of the said provision, in our view, will not depend upon the numerical count of such incidents or only on the continuous course of such conduct, but really go by the intensity, gravity and stigmatic impact of it when meted out even once and the deleterious effect of it on the mental attitude, necessary for maintaining a conducive matrimonial home. If the taunts, complaints and reproaches are of ordinary nature only, the Courts perhaps need consider the further question as to whether their continuance or persistence over a period time render, what normally would, otherwise, not be a so serious an act to be so injurious and painful as to make the spouse charged with them genuinely and reasonable conclude that the maintenance of matrimonial home is not possible any longer. A conscious and deliberate statement leveled with pungency and that too placed on record, through the written statement, cannot so lightly be ignored or brushed aside, to be of no consequence merely because it came to be removed from the record only.

Citations : [2003 SCC 6 334], [2003 ALLMR SC 3 777], [2003 AIR SC 2530], [2003 SUPREME 3 416], [2003 AIR SC 2462], [2003 SCALE 4 134], [2004 BOMCR SC 2 384], [2003 ALD SC 3 124], [2003 AWC SC 3 2101], [2003 BLJR 3 1658], [2003 DMC SC 1 685], [2003 JCR SC 3 1], [2003 JT SC 4 85], [2003 LW 4 609], [2003 MLJ SC 3 115], [2003 PLJR 2 200], [2003 SCR 3 607], [2003 UC 2 1211], [2003 UJ 2 947], [2003 AIR SCW 2530]

Other Sources :

https://indiankanoon.org/doc/1228342/

https://www.casemine.com/judgement/in/5609ade1e4b01497114126d8

https://www.indianemployees.com/judgments/details/vijay-kumar-ramachandra-bhate-vs-neela-vijay-kumar-bhate