I don't really understand the point of this question. What does mental illness have to do with whether a person can become a JW or not? Why are you putting people with mental illness in some kind of special category? If they for some reason did not baptise people based on their mental condition(and who is going to decide that?) then you would be hollering that they are discriminating.
Besides what are they going to do, tell baptismal candidates: "Prove you're not mentally ill!" ???
No Apologies