Here are the relevant quotes from the Canonical English version and the Retranslation:
"When the Mammon Machine was finished, the Queen changed almost over night."
"Since the Demonic Vessel was completed, the Queen has become cold, like a different person."
This line is from a Zealish person of no importance to the story. It seems pretty clear that Queen Zeal changed both considerably and suddenly once the Mammon Machine became operational. From my own studies in humanity, I am led to conclude that we can infer that she was particularly susceptible to the corrupting power of Lavos as expressed through the Mammon Machine. Whatever she had been before, she changed very quickly.
This doesn't necessarily tell us much about what her overall personality might have been like. It doesn't even tell us what the mechanism of Lavos' corruption was; only that it happened to Queen Zeal very quickly.
To speculate on Queen Zeal's actual personality, all we have is Janus' line:
"She's NOT our mother! She looks like mother, but inside she has changed."
"That bitch isn't our mother! The appearance is mother's, but she's something different on the inside..."
Sadly, this isn't very informative. All it seems to establish is that Queen Zeal was a more appealing mother before she changed, since Janus' emotions are betrayal, resentment, and disgust.
I don't think we can draw any conclusions based upon the ease with which she was corrupted. Queen Zeal's personality prior to the Mammon Machine seems almost totally open to speculation, although perhaps it does not reflect very positively on her that Janus turned out to be a spoiled brat and Schala turned out to be a bleeding heart to the point of knowingly imperiling her entire kingdom just to save a couple of pretty faces.
Personally, my wild speculation is that Queen Zeal's personality alteration was as large as it was swift...and that previously she had been a calm, kind, person. What her ambitions were, I couldn't say. That sleeping fellow mentioned that she had nurtured magic after King Zeal died. Who knows what that might represent? Someone well-versed in Japanese culture might be able to discern an implicit meaning here, but I certainly can't do any more than wild speculation, myself.
She is one of the more interesting characters in the game, although that probably tells us more about me than about Queen Zeal. She's really something of an empty slate.
