Thanos can make more, but he really shouldn't.
No "right" or "wrong"
First off, when looking at the problem of overpopulation, and this specific solution, there is no right or wrong answer. There isn't a catch-all solution that will work in anyone's favour. There is only what one chooses to do.
Similarly in the real world many problems have a multitude of possible solutions, each solution having its pros and cons and in the end only one of these possibilities is chosen and acted upon, carrying with itself all consequences of the choice.
Thanos could have just "made more" but he chose not to, many people in his position would have done the opposite, many others would do the same.
Why "more" is bad
The Infinity Stones are objects of great power, even more so, considering they are imagined in a comic/movie. But still, in order for anything, be it a comic book or a movie, to be at all relatable there must be at least some basic real world principles followed. The moment your object/character/superpower ceases to follow any rules, limitations or at least the very basic physical principles - you loose any relation to the audience and the whole point becomes moot. It would be like arguing why Salvador Dali's Clocks are melting, when they should be standing firm.
Having said that the one law of physics that always MUST be observed and that nothing can escape from, be it a star, a planet, an atom or even light, itself, is the Law of Concervation of Energy.
What that law says is that the amount of energy (and thus, matter) in the Universe is always constant. You cannot create or destroy energy or matter. You can only transform it from one form into another.
"More" means "less"
What this means is that if Thanos were to create more food for all people (assume I mean all sentient creatures when saying "people") he would have to take it from somewhere, in effect only hastening the diminishing of resources.
If you have more food you are going to populate more and faster, only bringing the problem faster and worse than before.
Running out of space
You could argue he could just make even more food every time. That would be worse and worse each time, for starters - you would soon start to run out of space, as in ground to live on (since you would increase your population even faster each time).
More food that just appears out of nowhere would crash every economy that has producers and marketers of food. Prices would have to drop to compete with free food "falling from the skies", which would lead to cutbacks, layoffs and in many cases the government would have to step in and limit access to this miraculous sustenance.
The problem becomes even worse when you take into account people need more than just food. The new populations born thanks to more available food are going to need clothes, jewelry, luxuries, etc. Giving them that with the Infinity Stones would diminish the Universe's resources even faster, crash the economies even worse and so on.
Excess is bad
Humans (and therefore all movie aliens) are built on the premise that there isn't enough and often there might be nothing at all. Our entire function, like any animal, is geared towards surviving the harsh world as best we can.
- When well fed animals will abstain from unnecessary actions to conserve energy.
- When animals find food they try to eat as much as they can to the point of vomiting.
- When any one of our basic needs isn't satisfied we are focused towards satisfying it. Conversely when a need is satisfied its safeguarding is usually forgotten or disregarded.
- The whole point of "ambition" is the desire to be in a better position in life, you can only strive towards a better position if your current one lacks something
Keeping all of the above in mind if Thanos were to satisfy all needs of all people there would be terrible social degradation. No one would work, nothing would get done, skills, knowledge and great works will be forgotten, society will devolve to obese, lazy creatures. Like a behavioral sink.
No good deed goes unpunished
Presumably the Avengers wouldn't want to stop Thanos if he wanted to create more in the Universe but every single major corporation in the entire Universe would. And that would be much worse than the Avengers.
Even disregarding that something very important has to be taken into account - Thanos isn't obliged to supply people with more, like said before if he were to do it once, he'd need to do it again and again and again... As we saw that takes a lot of effort and energy. Effort no one is going to thank him for, effort that will only be met with complaints and critique. In short - doing that would quickly make him regret it.
If you think that is far fetched, even our current daily lives are full of examples where people try to do good but are rarely thanked and abundantly critiqued for that.
Sheep in a pen
If you were a farmer with a bunch of sheep in a pen and a pasture and those sheep multiply, as animals do, to the point where the space in the pen runs out do you build a bigger pen and find a bigger pasture? Do you do this every time to the point where you need to spend 12 hours a day maintaining the pen and the other 12 herding your sheep across half the county to get them to enough pastures to feed them?
Or do you sell/kill half?
Now, the obvious argument some people would make is "But sheep aren't sentient.".
You only think sheep aren't sentient because humans are much more powerful than them and know a lot more than they do.
So in Thanos' eyes humans aren't sheep. Because sheep at least are useful.