Calligraphy - the art of beautiful writing - involves a fairly deep understanding about the nature of handwriting, and it's a relatively small hop from calligraphy (and the ability to identify "hands" and graphology, or the use of psychological analysis to understand the scribes.
To understand Trump's signature, it's first worth understanding that this signature is one of the most recognized in the world, as he uses it extensively in his branding. I suspect that he perfected it specifically to look forceful, confident, and domineering, though like so much else of what Trump does, there's a lot that leaks through.
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First, there are no loops. The letters D,o,n,a,d,J, r,u,m,p and all have curves or loops, yet in most examples of his signature, there is little to no curvature nor is there completeness. Curves are feminizing aspects, and the completely lack of them likely point to a fear of being perceived as anything but an aggressive, alpha male. There are also no flat surfaces anywhere in his signature. That tends to be a characteristic of people who do not think ahead, who look only at the moment.
The signature is tall, but the ratio of the x-height (the top of lower case letters) to capital letters, is comparatively large - about 0.6 to 1. Large capitals in general tend to indicate significant egos, but the smaller-the x-height ratio, in general, the more intelligent the person is. Trump's x-height is fairly large, meaning that he's a person of fairly average intelligence - not stupid, but not the genius he proclaims himself.
There's also very little indication of slant in his signature. Most right-handed writers have a right handed slant, with the tops of letters slanted about 5-10 degrees to the right relative to the bottom. When there is no slant, it's either a sign that someone learned to write on engineering diagrams or that the person tends to have trouble with empathy or is sociopathic.
The condensed overlap of letters and the lack of differentiation is also a sign of both haste and laziness.
It's also worth noting the fact that Trump prints rather than writes:
Here, you have no differentiation in size, and a tendency to obscure weakness by using big-blocky letters with little distinction between capital and lower case letters. It is very different from his signature, though notice again the lack of roundedness, as well as the lack of descenders. This is a person who does not like to show his inner-self, and tends to be very guarded.