The issue of uncertainty with nuclear use relates to the timing and necessary trigger actions, not whether they will be used at all. Potential enemies must be certain you will use nukes at some point, but not what that point is. Thus an enemy thinking of attempting to escalate gradually, or push the limits of what would cause a nuclear response would be less likely to do so if they could not be sure where the boundaries were.
I can't find the reference now, but I'm sure I read that it was official policy to portray the US President as slightly unstable for this reason. I'm pretty sure this started in the mid-1980's.
Of course, any propaganda is unnecessary if one's leader is
actually senile, power-crazed, or liable to lash out randomly under pressure. This has been the case for quite a lot of nuclear armed countries most of the time since the mid-1980's
The question of a nuclear response is constantly on the minds of Russia, Iran, etc. However they probably regard the current incumbent as highly unlikely to order a first use in any circumstances, which is actually more dangerous in the longer run than the prospect that he might.