When naming a character, is thematic naming or realistic naming more important?
Naming conventions are entirely discretionary and depend largely on authorial intent and reader demographics/genre expectations.
Serious literary works might place more of an emphasis on historical accuracy, for instance, so the author might avoid any sort of anachronistic nomenclature that undermines their work's authenticity. They might reproduce a realistic name based on the conventions of the culture, language, region, occupation, time period and social status of their character.
But realism doesn't automatically sacrifice symbolism and naming motifs. Many common naming attributions are already naturally denotative. Surnames can symbolize occupation (Smith/Tailor/Cotter/Carpenter/Butcher), region (lexical suffixes), family dynamics (patriarchal/matriarchal/tribal), or even status. Monikers, nicknames, aliases, pen-name handles and titles work in the same manner, they are naturally-occurring or culturally-appropriate aptronyms that reflect the personal attributes of the character.
The literary customs surrounding naming motifs are just as important and serve their own purpose. Often, these sorts of names hark back to a time when name-punning and alliteration were fashionable satirical devices used to ridicule, hyperbolize and poke fun. Charles Dickens and JK Rowling both capitalized on this sort of humor, allowing social commentary and stereotyping without being overly subversive. Naming motifs within certain genres, particularly the sub-types that fall under the umbrella of speculative fiction, are very popular, not only to uphold the whimsy of their respective genres but to add cohesion to the narrative by crafting characters that become an extension of the story's theme itself. This is a useful device if you're writing an allegory or parable, but it can toe the line of absurdism too much, destroying the realism that an author might have been aiming for. And if you're targeting a specific reader demographic and genre that don't favor the absurd, it will backfire. So you have to understand your audience's expectations.
But each serves their own purpose and aren't mutually exclusive. Again referring to JK Rowling, her Harry Potter series incorporated both philosophies for entirely different reasons. She used absurd names to denote foreign origins, to symbolize and to stereotype, to invoke wonder and satirize. But she also used realistic names to ground the reader and enable them to suspend their disbelief when confronted with the ridiculous.
Harry, for instance, is a very ordinary name meant to help a reader relate to the character because this character serves as a reader surrogate. The reader can more easily insert themselves in Harry's shoes if they relate to his status as an ordinary person freshly introduced to the world of magic, because each reader is, after all, a non-magical muggle from ordinary origins themselves.
Whereas other names, like Narcissa Malfoy, are obvious aptronyms meant to stereotype and symbolize her personality as a narcissistic maladaptive person... which is also a foreshadowing device since this character is a villain in the story.
So the importance depends on how the author intends to present their ideas.