I do believe human beings are smart in general for sure. But if technology is a scale then there are trek races on the lower end. In the scope of Star Trek, if one is using ancient tech then they are hopelessly outmatched at their present time. Many times they need outside help to even survive.
Survive what? Outmatched by whom? If there's a competition, the only ones competing with them are the aliens coming down talking about how superior they are.
Civilization isn't a race. There's no requirement that everyone has to catch up to the level arbitrarily set by the most advanced state -- not
unless that state is deliberately making things hard on the people who are less advanced. Europe wasn't harmed by China's considerably greater advancement because China wasn't interested in cultural imperialism, preferring to let others come to them and otherwise leave them to their own devices. But many cultures in South Asia, Africa, and the Americas were devastated by Europe's greater advancement because Europe made a "match" out of it, competing with them and insisting they had to catch up to "survive." The only ones creating a threat to their survival were the ones meddling in their lives.
There are smart people in every time period but everyone is chained by the time they are born into. A doctor using leeches vs today's medicine. Old timey dental devices vs. todays dentistry etc... imagine living through that time period and finding out that a highly advanced race chose not to give you the medicine that would have helped.
And that's the problem with TNG's take on the Prime Directive -- it obscures the real issues of well-intentioned cultural imperialism that made the PD a good idea in the first place. As I said, study your history and you can see the horrific damage done by cultures who assume they're helping.
Personally I don't believe in a hands-off policy. I believe that interaction can be healthy as long as the more advanced culture respects the other culture's autonomy and lets them control the interaction, decide for themselves how much technology to adopt and how to fit it into their lives. But there can be a slippery slope if you're not careful, if you fall into the trap of assuming that superior technology or power equals superior wisdom or right to decide. The Prime Directive is a safeguard against that kind of well-meaning condescension. I don't believe it should be an absolute ban on engaging with less technologically advanced cultures, but it's valuable for getting you to pause, think about what you're doing, and proceed with extra care and respect if you do intervene.
The original series came along at a time when the ugly impact of colonialism was just coming to be understood. The end of the Indian Raj had been less than two decades earlier, decolonization was going on in Africa, and Southeast Asia was being torn apart by the aftereffects of European imperialism. It was becoming painfully clear how much damage the "White Man's Burden" mentality caused, not only to the people the imperialists had tried to "help" or "uplift," but to the former imperialists themselves. The writers of TOS created the Prime Directive as a reaction to that, a recognition that well-intended cultural imperialism was a dangerous thing and that it was more important to respect the free choice of other cultures. It was perhaps an overreaction, but the concerns it addressed were very real. It's unfortunate that by TNG, people had lost sight of those lessons and reinterpreted the PD to mean "They're so primitive and stupid that they can't possibly survive our superior knowledge," which is just the flipside of the same condescension. It's still assuming you're entitled to make decisions for them rather than respecting their right to make decisions for themselves. And since the PD has come to be associated with that unfortunate idea, it's obscured the merits of the original, anticolonialist version.
People may have been smarter at math in some ways back when students were denied calculators in class, but we are still more advanced than them in many ways. Isn't that why people discover things & invent them, so future generations can stand on their shoulders and improve on it and move things forward.
But what if someone more advanced comes down and hands you all those things so that you never discover or invent them for yourselves? Will you really understand them and be able to use them wisely? Will you be able to build on them, or will you just be dependent on your mentors to hold your hand forevermore? It's the "give a man a fish/teach a man to fish" principle, basically.
In any case, the important thing is that the decision should be theirs. It's their culture being affected, so it's their responsibility. You can't possibly understand that if you insist on thinking of them as "lower."