<Nitpick mode on.>
It seems the average IQ of the US is 98, not 100. But even if the mean would be 100, and <100 would be "dumb", it does not follow that 50% of the population is <100.
Actually, since IQ is bounded on the low side and not bounded on the other side, it is actually likely that if the average is 100, more than 50% are below average. But that is not guaranteed. You could have only one dumb person with everyone else >100.
IQ is well known to be normally distributed. One property of the normal distribution is that median = mean, so it follows that less of the population would have an IQ below 100.
Still nitpicking:
Since IQ<0 do not exist, it cannot be a true normal distribution.
It is true that IQ distributions over large groups resemble normal distributions in their core, i.e. close to the median.
Actually, since IQ is bounded on the low side and not bounded on the other side, it is actually likely that if the average is 100, more than 50% are below average. But that is not guaranteed. You could have only one dumb person with everyone else >100.