Were the Japanese Engineers Who Built Fukushima Incompetent?

To the best of my very limited knowledge of nuclear plant design - no. Insufficiently paranoid and not gifted with perfect knowledge of the future - maybe.

The facility was fairly hardened versus geology as it was. It's just that geology is as big as the planet while anything you can possibly build is taken from scraps laying around on a tiny surface layer. It can and will punch you in the face despite your best efforts to plan for it.

It was also built starting in 1967, so we are talking fairly old designs. Three mile island was 1968 and related to bad interface design. Chernobyl was 1970 and related to it being designed and built in the USSR. Modern designs are light-years ahead of these, there's just too much paranoia and FUD to get money to implement them.