Does this phrase represent an actual clinical diagnosis?
There is no such diagnosis in the DSM. Based on your definition (the tendency to start new ideas without thinking them through), it is probably correlated with psychological constructs such as impulsivity or deliberation. These can be measured using tests such as the Barratt Impulsivity Scale and Cognitive Reflection Test, respectively. Whether or not people who fall prey to "shiny object syndrome" in business are actually more impulsive or less deliberate is not clear.
Are there people who are more likely than others to be distracted by "shiny objects"?
Since "shiny object syndrome" hasn't been operationalized, it's impossible to say for sure. I think it's reasonable to believe there is individual variation here, either because some people are more cautious than others, or because they have learned from their mistakes.
Can "shiny objects" cause further effects other than distraction?
Sure, why not? By definition, it sounds like there would be a drop in efficiency or productivity, which can at least cause negative monetary outcomes.