Specifically, if you have two people who read a work, they may have different opinions on what "the theme of the piece" is. This is normal in the world of literary analysis.
Of course. That's par for the course in any artistic endeavor. It doesn't mean there is not a strong central theme. It means that the thing is multifaceted enough to present many angles for it.
Which is key in artistic analysis, but doesn't have a lot to do with the D&D game, by and large.

A strong theme can improve the often-ambiguous narrative element of D&D (if you want a strong narrative element; many games do not). It can work in an over-arching campaign, in a single adventure, or whatever.
Umbran said:
My literature teachers would not call "destroying evil" a theme. It is a very general direction of plot, which is by no means the same thing. "Theme" is a broad idea, lesson, or message conveyed by a work.
"Normal people have more strength to resist evil than you'd expect, but in the end evil sows the seeds of its own destruction" is a major theme of LotR - represented by the basic actions of Sam, Frodo, and Gollum.
I think it's safe to assume that "theme" is being used in a very broad context in this conversation, rather than the specific, narrow, academic context.
That said, I'm hardly a LotR scholar, and I'm entirely sure my off-the-cuff "theme" for it could be improved.
The crux is, "Normal people save the world," and "evil kills itself" are entirely possible to weave into a D&D game, complete with mechanical, statistical, possibly even numerical, representation to reinforce that theme through more than just the DM's say-so and the players' complacency.
And doing so enhances the strength of the narrative you tell at the table.
If you're not looking for a narrative (if you're more of a sandbox or a dungeon-delve kind of group), it's kind of pointless, sure. Though I tend to think composing a narrative is one of those things that is uniquely suited to TTRPG's, and that videogames deal with the other two play styles better.