Mentally stable or not, I wouldn't trust him in the white house. POTUS is among the most demanding jobs and requires a multitude of decisions and is imaginably a huge load of stress both from the pressures of the job and the constant thick skin required to get through constant criticisms of policy or an expectation to do things right. Trump has usually failed at the point of keeping a thick skin and he's mainly pulling at populist rhetoric without the slightest registering of how reality would play out. But whereas I criticized Obama on this in 2008 because it was largely idealist policy he likely knew wouldn't get passed so easily, Trump just plain doesn't seem to know what he's doing... or rather I think it difficult to understand if he'd actually do it given his penchant to change positions on a dime.
Irrespective of what it is, his argument boils simply down to rally the populist, anti-establishment far-right. I don't mind being anti-establishment, but his populist rhetoric leaves much to be skeptical about and he has a foreign policy lean to match.