I don't see the issue with people questioning the appointment. It always seems odd to me when people go excessive on the congratulations for a final, as if they know the person or the person is likely to read it. If Simon Hooper is appointed to Chelsea V Spurs this weekend in the PL then no one is going to congratulate him but many would question it, so why should it be that much different for a final? In many ways, the more important the game, the more scrutiny there should be.
When Steve McClaren was appointed England manager, was no one allowed to qustion it because he'd been a manager for quite a few years and it was a big appointment for him?
For what it's worth, I have mixed feelings on the appointment. Lee Mason is an experienced PL referee and I think has had a solid season so it is reasonable that he should come under consideration. But equally I don't think it's wrong to question if he has been fortunate to spend 15 years or so as a PL referee or to ponder if this reflects an overall decline in the depth of refereeing at the top level.