Very short answer: it was common and expected.

Long answer: see below.

French literature is rooted in old principles, dating back from the mid-12th century at least. A story depincting upper-class people has to display a refined language for the language mirrors the nobility of the characters. It works altogether with the genre. A tragedy written in a fine language was the only socially acceptable way to portray the nobility for a very long time (at least, in theory).

It's already implemented in Medieval novels. Early authors who'd narrate the story of long forgotten kings argued that they did such in a language that honored the characters they chose to present. It's been observed however that they often took a few liberties and toyed with their mighty principles on occasions to make their stories more entertaining, etc.

When genre really became a proper literary construct--because they weren't, really, during the Middle Ages--authors stiffed up even more. Racine's tragedies share nothing from the comedy we witness in Shakespeare. A tragedy was the only proper set to display aristocrats. Other members of the society were shown in comedies. They had to be well-written too, of course, the French language is called "la langue de Molière" after all, but Molière played more with the language than Racine. And he could because he was mostly mocking the bourgeoisie. The language, the genre, the characters, it all fitted together under a single paradigm.

Then came the French Revolution and the institution of national education. Come the 19th century most people could read in France. Plus, literature wasn't the business of patrons anymore. Before the Revolution, authors had to find a protector to finance their writings when they couldn't live off their own estates (like Ronsard or Montaigne did). However, French aristocrats became kind of a rarity after so many of them were "guillotined" and they certainly were in no position anymore to finance or to protect the arts like their ancestors did. It took some time too for the bourgeoisie to restore that tradition. That's exactly when author's rights were implemented.

Pierre Augustin Caron de Beaumarchais, adventurer, arms dealer, philanthropist, and many other things, got mad when he saw his theater piece "Le Mariage de Figaro" being performed all over in France. Why? Because theater directors were making money and he wasn't! He pleaded to the National Assembly and eventually won, starting the very long fight of author's rights. Indeed, an author could be protected in France but who was there to stop any Belgian publisher to live off his writings and publish them freely? Nothing, until diplomatic arrangements were done later in the 19th century.

An author, however, could now live from his writings. It was a first! It had never been the case before! Plagiarism hadn't even been a concept until then, or barely. Therefore novelists were not writing to please the court or the high standing society anymore. They had to sell their stories to the masses! They had to tell the stories of underdogs and common people. The Three Musketeers is very anti-establishment narrative when you think of it. D'Artagnan is penniless at the start of his career. He battles his way to the top by fighting off the Cardinal and his guards. His friends and him have servants who speak in the most crude manner (for the time). Émile Zola, in a totally other genre, also tells the stories of oppressed and common people: at the opera (Nana), in the mines (Germinal), in shopping galleries (Au Bonheur des Dames). In those stories, the people in power are very often displayed as ridiculous, offensive or rude. Only a few historical characters find the graces of 19th century novelists, like Napoleon or Louis XIV.

The Three Musketeers were written by Alexander Dumas, so was the Count of Monte Cristo. Guy de Maupassant didn't diverge a lot from the pattern. "Bel Ami" tells the story of a greedy social climber who lacks both class and finesse and therefore makes it to the top. Flaubert, Stendhal and the lot, they all loved ordinary people as main protagonists and they loved to put them against the establishement. We also find that in the English literature: "Oliver Twist" and "David Copperfield" jumps to mind. Novelists dared telling stories that were unheard before because their audience was drastically different than the one of earlier writers thanks to mass literacy.

It came at a price though. Zola and Maupassant liked to print dialogues that mimicked the oral language. The Academy took it as an offense. Books were to uphold a higher standard of language than what was spoken in the streets or in the fields by common people. Dumas made several times a fortune with his novels however he was never appointed to the Academy. He never got a seat. He never was called "an immortal". Nevertheless everybody remembers his name over those of the people who sat at the Academy--unless we take Victor Hugo into account, of course.

And what is Hugo's greatest success? "Les Misérables"...

There was certainly a great divide between the common people who read novels and the upper-class who didn't see fiction writing as a serious activity. They rather delved into law, science or history books. When fiction writing became a respectable "profession" by the 20th century, we still had French author who prided themselves in not being understood or enjoyed by low-class readers. Such was Julien Gracq. He didn't care if he didn't sell many books as long as only upper-class and highly educated friends would enjoy his novels.

To conclude my answer I'll also add that Edmond Dantès (the main protagonist of the Count of Monte Cristo) lives up to the highest standard of virtue. He litteraly embodies the idea of Justice--though you may not like how justice is done. He stands tall, deeply rooted in his moral values, whereas his opponents only believe in profit and money. Commoners may be poor (unless they happen to find the secret island of Monte Cristo), even Musketeers, but they certainly hold the moral high ground when it comes to it and that's what makes them the heroes in epic historic tales such as those written by Alexander Dumas. Zola, Flaubert, Stendhal and Maupassant were less interested in heroes so... their characters deviate from that pattern.