Not really. Our offense this year was going to be limited no matter who the coach was. Just like 2017, we had enough raw talent in the running game that we could beat mediocre defenses solidly, grind out points against good defenses, and be completely shut down by elite defenses where you have to have a little bit of a passing threat. With Mullen, we had six locked up, and we might have ended up dropping Auburn or A&M, but we would have been able to move the ball some against all four "toss-ups" and also LSU, so it seems likely we would have had a net gain, especially considering how minimal production we needed for UK and Florida. Certainly no guarantee, but if he didn't win 9 regular season games, it would have been because Grantham was a bid downgrade from Shoop.
I don't know if they'd have been happy, but they'd be less concerned. Struggling with a new offense is fine. Looking completely incompetent is worrying. Also, it's a lot easier to recover from making a bad DC hire than it is to recover from making a bad HC hire. If shoop hadn't turned out well, we could have gotten rid of him after 2019 (or even this year; Mullen basically encouraged Torbush to look after year 1). If Moorhead turns out badly, it's much more painful to get to and through a transition.