Less bad but still bad. Consider that /bin/mkdir is still interpreting whatever you pass to it. What if you provide it a "filename" that starts with a dash? Then /bin/mkdir will interpret that as an option. What if I give it "--version" as the name? It will return 0 (success), even though no directory was created. While it is rather harmless for /bin/mkdir, this might be a bigger problem for other programs.

There's not much context in the story, but I would guess you might want some sanitizing for stuff like ../../../<whatever>/

No, not really.

You might need some additional checks, like for ../ and symlinks, but you need these regardless whether you execve or write your own mkdirp().