The theory doesn't make any sense; unlike The Sixth Sense, the main character in Interstellar interacts with a lot of people. If he's dead, so are they (or they're just figments of his post-death experience).

The journey doesn't make any sense as a post-death experience, since none of the events are tied to his life (before the events of the movie), so there's no deeper meaning to the wave planet, or the ice planet, or falling into the black hole.

Also, Nolan doesn't do meta-contextual films. His specialty is the technical aspects of storytelling (see, e.g., Memento, The Prestige, Dunkirk, and Tenet), not in imbuing deeper emotional meanings into his works. Indeed, one of the big critiques of his films (excluding Dunkirk and Oppenheimer) was that they generally lack any emotional depth.