Useful idea. I sometimes use time [1] to print stats at the end of the program's run, like so.

    /usr/bin/time -f 'user=%U system=%S maxrss=%M ins=%I outs=%O" PROGRAM

Seeing stats while the program is running is nice though! Also, cumulative monitoring of an entire foreground process group would be super handy (a long build, for instance).

Drop the --target-output option, you don't need it. Same thing can be achieved with:

   monner.py sh -c 'exec 1>/dev/null 2>&1 $0 "$@"' PROGRAM

I also agree with the other commenter that monner should be the parent process.

[1] Note, time(1) is not the same as the shell builtin of the same name.