I'm from Glasgow and have family from all over Scotland (and England. Some in Canada and Australia too... it's a big family).
Cait Sith does sound kind-of Northern Scottish, but it's off in some places. Scottish accents can be confusing to someone that's on the outside. The accent and dialect used depends completely on where you stay, and varies from region to region (sometimes these regions can be really small). For example, Glaswegians (me and other people from Glasgow) have a Glaswegian accent. It sounds a bit rougher than most other Scottish accents (we swear a lot more when we talk also). Edinburgh is a 40 minute train journey away, and sometimes Glaswegian's can experience some hostility when they go there. The common nickname for Glaswegian's around Scotland is "Ouija's" (as in, Oujia Board) because of the "Wegia" part of "Glaswegian". You sometimes find it being used in an offensive way (especially in Edinburgh, while there are lovely people there, a good few in my experience seem to think that someone from Glasgow is the shit on their shoe. This attitude is encountered a lot).
My point is, the Scottish accent is very varied. The Motherwell accent (though not very defined) is very different from the Glasgow accent, which is very different from Edinburgh, very different from Aberdeen, very different from Inverness etc etc. The Scottish Islands (i.e. Arran, Bute, Cumbrae, Stornoway, Orkney, Shetland etc) all have their own accents too. Hell, literally a 20 minute walk from my flat and you're in Paisley - they have a different accent.
Example:-
Written - (My father's having a party on saturday night)
Spoken (Paisley) - (Ma fai-thurs huv'n uh pair-eh oan sai-urday nigh')
Spoken (Glasgow (slang)) - (Ma da's huv'n a par-eh oan sa-ur-day night)
The point is, when people try to sound Scottish on the media and they're supposed to be from a specific region (See Desmond from Lost and a character from Alias that was in one episode. My god that was awful, tbh, Desmond's accent is quite well done, however I remember in this particular Alias episode the brother and sister were supposed to be from Glasgow. The guy had an IRISH accent and ties to the IRA. How the hell is that Glasgow!? Grrr. The girl sounded like she was from Aberdeen.)
Sorry about the massive speil, I just need to vent my frustration about this. They always get it wrong. In the case of Cait Sith, I don't really know. It's definitely Scottish, but it doesn't sound like any region i've ever been to. And yes, I've been to the highlands.