Donald Trump has proven throughout the election campaign that he is a petty man, often acting like a petulant child. Therefore, it is no surprise that he has made these comments. They are damaging to the image of the U.S. which is seen as the biggest pusher of democracy for countries around the world. But Trump might make them think: Why would we want democracy if the U.S. can't act in a democratic way?
The world looks to the United States as the premiere example of democracy. They may see Trump as a very successful business man, who won a major political party's endorsement, and give his claims credibility. If the world begins to doubt this established political process, what other things may they bring in doubt?
If Trump really thinks the election is rigged, some people will believe him. More importantly he is not respecting the democratic process as it exhists and is whining like a child who is losing a board game and knocks the board over because he or she is losing the game.
Most of the rest of the world gets to see Trump through the eyes of their national media. In general he is portrayed as a loud-mouthed buffoon who has sexually assaulted women, has failed businesses and is a sore loser. People are more likely to believe his claims of a rigged election are pure fantasy than feel that U.S.democracy is diminished.