I haven't actually seen Moses Jones. I've seen it advertised, but it doesn't seem to be the kind of programme that appeals to me very much. So I can't honestly form an opinion on Matt Smith's performance in it.
I have seen Matt Smith in a number of other things, however. The Sally Lockhart mysteries - "Ruby in the Smoke" and "Shadow in the North" (where he had a major role playing Billie Piper's sidekick) for instance, and he was excellent in those. To be honest, I'm kind of worried whether a 26 year old actor can pull off the role of the Doctor, but if anybody of that age can it's Matt Smith.
Also, I trust Steven Moffat. If he reckons Matt Smith is the best actor for the role of the 11th Doctor, I would be very surprised if he's wrong. Who can doubt the man who wrote stories as brilliant as "Blink", "The Empty Child / The Doctor Dances", "Girl in the Fireplace" and "Silence in the Library / Forest of the Dead".
Anyway, if your opinion that Smith isn't very good in "Moses Jones" is correct, but the fact remains that he has been very good in other things - that means his performance obviously depends on the role, and we'll just have to wait and see how he does as the Doctor.
A lot of people had similar worries when a little known actor called Tom Baker was announced as the new Doctor in 1974 (he was actually working on a building site when he was chosen to play the Doctor, and at the time was the youngest yet), but he turned out to be one of the best, and a lot of people's all-time favourite. So lets give Matt Smith a chance, and actually see him playing the Doctor before forming an opinion.