But penalizes the iconic tiefling cultist of the fiendish demon-god. The problem is that no matter what mental stat you decide to penalize, you're going to be nerfing at least one really cool iconic bad guy role.
I can see where you're coming from with the multiple bloodlines, and indeed for a planar campaign, I think you probably need them. The central issue is not nerfing bad guys, though, it's trying to present the race coherently. +2 Dex, +2 Int, -2 Cha has history behind it, but maybe a little too much history. I'm going to just say I don't think most players should be directed to "The Tiefling Issue" in order to get a tiefling that covers basic archetypes.
As it is, fiendish sorcery is a kludge, and essentially one that punishes arcane tiefling sorcers.
Looking at the iconic through a different lens,
1. Why can't the cultist be a sorcerer?
2. -2 is not fatal for the one-off tiefling. As it is, the tiefling arcane sorcerer has to deal with the same issue and I am more concerned about him.
3. I can see the tiefling cultist as an archetype, but it's far less compelling to me than the tiefling Sigil rat, tiefling caster, tiefling arcane trickster, tiefling musician, etc. Tieflings to me are the downtrodden masses of the planes and the transgressive things lurking at the edges of major cities on the Material plane. They are grifters, "helpful guides," disposable spear-carriers, and living currency. Not necessarily big cult leaders. Or even particularly clerical acolytes. "Cultist" translates to me to "rogue who got religion."
4. I'd be more inclined to make a clerical leader as a cambion, or whatever kids are calling them these days.
5. Hello, advanced template.
You could turn a lot of that on its head, but at the end of the day, to me tieflings are adept at arcane magic and roguery, and they are rarely clerics. Even diabolical ones. They don't worship, they
serve.
If you really wanted to play with the idea of tieflings coming from different bloodlines, or blended ones, they should probably have gotten the floating +2 that humans, half-elves, and half-orcs get. Particularly if you want them to hew to the concept of being part-humanoid, part-outsider, rather than "human with a twist of evil lime."