i don't think it would. in fact, in a medieval society, the royals would want to increase the amount of commoners to increase the "production" of a cheap work force they can use for labor. commoners were barely paid any money so they were super expendable, and the royals would just work them to death and then hire more. no sane royal in a stereotypical medieval european society would want to decrease the number of commoners - it's like with the "one percent." you have a small bunch of people with the power, and the majority of others are forced to basically sell their souls for a chance of surviving.
they would also want more commoners to tax. basically more commoners = more money and more workforce. less commoners = more equality between the royals and the commoners as it would force the wealth divided between all of the commoners to be spread between less people, making every person get more.
i think the royal suggesting they need less commoners was just an idiot who doesn't understand how he gets money - from taxing all the commoners in his domain.
Yes, true in normal circumstances they would. But if the commoners became more independent and started standing up for themselves they could be threatening to nobility. Setting examples from the populace, causing problems that only the nobles know how to solve, or just wiping out the village which was the main cause as intimidation or blaming it on someone else...
That can happen
But that only really happens when they are entirely pushed off of the edge.
Take for instance, the French Revolution.
The French Revolution occurred for two primary purposes: The United States refusing to pay back France for the debt during their revolution, and the previous king using taxpayer money to build Versailles, leaving them in debt.
In most cases, so long as your country isn't in debt like France was (which could have been avoided if the United States actually paid them back after getting their independence like promised), then royals want to have a high population of commoners and a very divided caste system to get more money. More population also means more people to send to war and/or religious crusades and massacres, so that would also be beneficial if the country wanted to expand their land to get more resources to make more money and trades.
Unless it was a very specific group of people, such as an ethnic group or religious/philosophic group, there would be no need to want to lower the amount of commoners to "protect your country's image," which is usually decided based off of royals and royal architecture, religion, and trading/military power.

"we need to reduce the percentage of the commoners" ...whut... I didn't expect a super intelligent webtoon but that's such a stupid argument :')