Like many of the comments here, I firmly believe that mental health support and measuring it are super subjective, so making a big deal out of a score on a table isn't necessarily representative of the uni's actual support available.
The league tables as a whole are pretty difficult to ascertain and definitely shouldn't be used as a sole form for university decisions; the best way is to interact with the uni and visit it.
Having said that, you could argue that if mental health is put on the tables, universities might spend more to support it and raise their standards as a whole.
It is important to acknowledge the importance of mental well-being to students, but arguably no more so than physical well-being and various forms of health.
Yeah, I'm not really sure, but personally, I think no, it shouldn't be included on league tables, however, I don't particularly use them when looking for unis.