Is Mister X worth hiring, despite mediocre stats, in Game Dev Story

Review scores don't seem to affect your chances; only the game's stats. Also possibly the game's combination, but you should never make a game that doesn't have an "Amazing" combination (more $!). I'm not sure whether or not creativity has a bigger influence on your Grand Prize chances than the other stats. Also, review scores don't mean anything anyway. If you save your game right before you kill the last bug, and you get a review score of 37, restart the app. You might get a review score of 36. Restart again, you might get a review score of 40.

You were correct in your own assumptions:

• Job Title is there for a reason! That person is good at a specific task, and should be used as such. However, the same worker should never be used on two games in a row.
• Choosing a worker with a high stat for a task he wasn't meant to do (IE: Choosing Sound Manager for Writing) is never worth it. It's not possible to get enough stats to make up for the deficit. The worker will be unfocused and perform poorly.
• Hackers should never be left as Hackers. Buy a Career Change Manual from the salesman and give them a role based job. Yes their stats will drop slightly, but it will still be worth it.

One of the best guides I found for the game while I was playing it is this one. Most of these points I learned from that site, and they cover a lot more as well.