Tennant, who is Scottish, doesn't have the thickest accent, but in the clips he is laying it on a bit thicker by exaggerating some (rather standard and cliched) sounds. There are a variety of Scottish accents that differ based on region, class, and education. The Scottish accent used most in entertainment is a thick and often put-on Glasgow-esque working class accent. However, that isn't how all, or even most, Scottish people speak, based on my study of dialects and accents in my undergrad as well as studying in England, Scotland, and Wales for several months. Believe it or not, Tennant's is a bit more authentic and tad less cliched than Angus Young's accent. Young spent most of his childhood in Canada and all of his adulthood between Canada and the States, though his parents were Scottish.
I am finding that Tennant's higher-pitched voice is throwing me more than the accent. I think of Scrooge McDuck as a deep-voiced, growling old curmudgeon; not this energetic, highly expressive middle-aged dude. Different takes on the same role may be equally valid and entertaining, though. I'll likely get used to it in time.