There's a strong chance that this would indeed be the case in the short term, but not necessarily in the long term.
Studies all point to intelligence being a genetic factor. In other words, our potential has some built in limits. The people being sent to Mars are probably above average in many ways. However, not all those ways are necessarily genetic.
You see, your genetic potential also needs an opportunity to be reached. You need a society which opens the doors to you getting a good education, and discovering those area you shine in. Some people are great athletes, as well as intelligent, but only pursue a career in sports. Others have a potential to be quite strong, but enter academia, and never develop a habit of working out. There's many possible combinations.
The bunch being sent to Mars are interesting because they no doubt meet quite high intelligence standards, but they must also meet certain physical standards as well.
And so, a very important question is just what balance was struck between intelligence and physical fitness. Some compromises might have been in made in order to mitigate a lack of fitness in very intelligent individuals, or a relatively lower intelligence in the strong individuals who have certain key skills the future colony will need.
Once the colony is established and the first colonists land, they will soon discover that their lives will now be a lot more difficult than they were back on Earth. Children born there will (arguably) have access to fewer resources than our children will. Will they really be smarter?
They may have a genetic predisposition to be more intelligent than your average Earthling. However, they will also live in a much more dangerous and stressful environment, where they might have a lot less opportunity to innovate, and generally reach their full potential compared to a child of similar intelligence here on Earth. This is because their existence will be a lot more focused on survival than on simply developing their own potential.
Also important to keep in mind is that eventually it will become economically viable, or even necessary to build up the Martian colony. By far the fastest way to do that is to send specialists from Earth over there to help them expand (rather than wait for several generations to grow up).
And so, when this new workforce is sent over there (some of them perhaps only temporarily), will the intelligence standards for them be quite as stringent as the ones for the initial colonists? Will all the future settlers and colonists?
Most likely not, as there will be a pressure to colonize, and if everyone is super intelligent, then who will want to perform the menial labor type of jobs? Not the professor level educated engineer.
In the short term the colonies will be populated with very intelligent individuals, however over time this gap will most likely narrow significantly.