A word of caution here.
It is easy to fall prey to making assumptions based on generalizations, which would be a horrible mistake. There are those with mental illness who cannot manage in marriage, and it is an aveiroh to allow them to marry. Poskim have spoken about this, so I am not out of line with this pronouncement. There are many who have a diagnosis and are in treatment and currently stable. I vote against keeping things secret. Yet, if the other side knew a diagnosis, they would run away, even if treatment resulted in a fully functioning individual whose marriage capability was no different from anyone else. So I vote against keeping it secret, but suggest that a decision be made on the evaluation of the individual and their condition, not a label.
A simple piece of advice to the secret keepers would be, “If the shoe was on the other foot, would you be outraged from the other side keeping such information from you? If so, don’t keep it from them.”
There are ways of presenting the information. The dishonest ones are recipes for disaster. I have observed a few such situations. There was an article in one of the frum magazines several years ago that described such a situation. It was awful. May we never know of such problems.