You seem to understand "plot" on very superficial level. A book does not need a complex plot to be great. On the Beach by Nevil Shute is another one of my favorite novels, and on a surface level, it also doesn't have a complex plot; rather, the character interactions make it great.

The exploration of ideas is the actual substance, and doing that through characters that represent ideas, ideals and ideologies is exactly where the novel shows its strengths as an art form. Simply writing a sermon about your ideas is what would be lazy; there's nothing lazy about TMM unless your understanding of plot is simply that "things happen".

Different novels have differents approaches to tackling their themes. In The Magic Mountain, Mann made the sanatorium and its patients into a metaphor for pre-WWI Europe, and at the same time, the book serves as both a classical Bildungsroman (with Hans Castorp learning all sorts of things, from falling seriously in love for the first time to witnessing the ideological clashes between Settembrini's humanism and Naphta's radicalism) and a sort-of parody of a Bildungsroman, as well as a subversion of it (Hans, after all, doesn't change much and, instead of growing up and finding success, >!enlists in the army and possibly dies in the war!<).

It also deals with how we perceive time and its passing; Hans' first three weeks in Berghof are meticulously detailed over the course of the first 200 or so pages, but then, as he loses track of time and years pass, the level of detail goes down, and leaps forward in time become more frequent, so that the reader too ends up unsure about how much time has passed.

I feel The Magic Mountain is one of those books that, just like The Brothers Karamazov, only benefits from re-reading. Mann packed it so full of ideas and themes, there's always something new to glean from it, just like with his other works, such as Joseph and His Brothers.