I agree that Carrey is miscast and that the whole thing winds up feeling like a vanity project but really the problems run deeper.

in general I can't vibe with any movie about some suburban white guy who feels "trapped" by his good job/many friends/smokin hot wife. when you really think about it so much of the movie revolves around Truman trying to cheat on his wife with some broad he knew for 3 seconds in college and we the viewer are supposed to be rooting for him. no bueno.

there's a very similar short film from the 60s that I think is directed by Paul Bartel, much better execution of a similar concept. Al Brooks also did a movie like this but I've never seen it.