I think Erika's stiffness stems from something much deeper than plainly being 'boring' or a 'bitch' and this issue may be alluded to later, within the scene from the season's trailer, when she says: "you don't know what I go through every night" (or something similar). This persona of Erika Jayne, as great as it is, seems to be who Erika inspires to be, whilst Erika Girardi can often come off as being very hollow and flat. But I do think there's a reason for this split persona/s, I've witnessed this trait in people before. I feel Erika is lacking confidence or inner validation and so uses Jayne as an outlet to channel/express something deeper (anxiety? addiction? who knows?), but truth is she can't be on stage all of the time. So I feel Erika is left trying to figure out how to act in social surroundings and live up to her stage persona when she doesn't have that outlet to channel emotion and rage into and struggles, so detaches herself from her surroundings therefore coming off as quite cold.
I sound like a psychologist hahahaa but that's just my two cents. However, after the season's trailer, I certainly feel Erika will upon up more much later on and may give us a closer insight into her personal life. Possibly (and this is a reach) she could be the rumoured housewife who is addicted to prescription drugs?
I don't know about the addiction stuff and lacking confidence, but I get the gist of what you're saying. From my perspective, I don't see it as negative traits or signs that she has bigger issues. That's just how some people are. She's a guarded person and she knows how to separate and compartmentalize herself. Personally, without knowing what's going more than anybody else, I respect that about her. It shows she's smart and in control of her own self. People saying that as something against her kind of baffles me!!!