I mean, it literally is an isekai story because the protagonists are taken to a different world outside of their control. It's up there among the most famous western isekai stories, with the likes of The Wizard of Oz, Alice in Wonderland, and Space Jam.

Only writer I know who didn't like Lewis was Philip Pullman, creator of the His Dark Materials series. Pullman didn't like Lewis including Christian themes in the Narnia series (calling them "religious propaganda") so HDM was basically the "atheist response" to Narnia.