Sentences in which event A happens
風呂からあがり、パジャマに着替えたまる子の口がポカンとあいたのは、それから一時間後のことだった。
Sentences in which event B happens
What event does それから refer to?
Is it saying that the time when Maruko got out of the bath etc was one hour after event A?
Or is it saying that event B was one hour after Maruko got out of the bath?
I'm sure its not ambiguous but can you please explain why it must be the way it is? If I had to choose, I'd go for my first interpretation, but I can't convince myself either way.
To work from the information given both in your question and comment above, the following could logically be said:
"What event does それから refer to?"
「それ」 must refer to "event A", to use your own words.
「それから」 = "since event A (occured)"
"Is it saying that the time when Maruko got out of the bath etc was one hour after event A?"
Exactly.
Notice the set of actions 「[風呂]{ふろ}からあがり、パジャマに[着替]{きが}えたまる[子]{こ}の[口]{くち}がポカンとあいた」 is nominalized by 「の」. Since it is nominalized, one can say that is 「一[時間後]{じかんご}のこと」, which is another noun phrase.
There has to be a time lapse of one hour between "event A" and the time when Maruko got out of bath, put on clothes and dropped her jaw.
"Or is it saying that event B was one hour after Maruko got out of the bath?"
No, that is just not possible.
It says nothing (unless you are not telling us something) about the length of time lapse between Maruko's after-bath set of actions and "event B".
To recap:
Event A occurs.
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(One-hour time lapse)
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Maruko gets out of bath, puts on clothes and drops her jaw.
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(Time lapse of unknown length.)
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Event B occurs.