This is based on a common type of phrase:
Show me a {description A} and I'll show you a {description B}
where the implication is that "If you bring me something that meets description A, I will be able to show you something that meets description B, because anything that meets description A also meets description B".
One example (from quick googling) is:
Show me a man with a tattoo and I'll show you a man with an interesting past
which means "All men with tattoos have interesting pasts."
So, from Trump's example we have that anyone who is a sore loser:
a person who becomes very upset or angry when he or she loses a game, contest, etc.
is also a loser:
- a person or thing that loses especially consistently
Interpreting the phrase "show me a sore loser, and I'll show you a loser" is difficult, however. It could mean:
Someone who has just lost and has justifiably got angry about it is still someone who has just lost
which implies "any failure is a failure, however you take it. i.e. any failure is bad".
Alternatively you could take Trump's phrase to mean:
"anyone who gets upset when they lose is someone who is unable to succeed".
Flipping this around, I interpret the phrase as meaning:
In order to succeed you have to learn to cope with the times that you fail
Side note:
My googling for similar phrases suggests that a much more common form of the phrase is "Show me a good loser and I'll show you a loser", which is pretty close to the first interpretation above of Trump's phrase. It implies that any failure is a failure, and there is nothing to be gained by learning to cope with failure.