The argument from economic theory is that if the fiat minimum rate is higher than the value that some workers can generate with their skills and labor, then those workers will tend not to be hired. Accordingly the general rate of unemployment will be higher than it otherwise would be without the interference of a law.
OTOH, if the fiat minimum wage is set below the entry level wage for minimally skilled workers, then it has no effect at all. Suppose there was a law stating that no one was allowed to work for less than ten cents per hour— would this law have any effect in reality? Of course not. No one would attempt to pay a wage that low and if they did, no one would agree to work for such a pittance. Almost by definition, a fiat minimum wage will be either higher or lower than the “real” value-generated minimum wage, because even if the law starts out generally “close to right”, the real minimum wage changes over time and the fiat wage will not.
There is another argument made about a fiat minimum that is “too high”— it sort of “cuts off the bottom of the economic ladder”, so to speak. People starting out cannot get the job experience and learn the skills needed to advance. If you are a marginal worker starting out just below the value that you need to generate to get a minimum wage job, you can end up “trapped”— no one will give you a chance to get some job experience and "OJT", so that you can advance to the next level.