You have the right sense of the meaning, but do you understand grammatically what's happening here?
This isn't a set phrase like ～のせい. The の is nominalizing 魅力的過ぎる, and the resulting noun phrase is being marked as the subject of いけない (is no good, is what's wrong). "It's that よしのん is too attractive that's no good", or in natural English, yes, "It's よしのん's fault for being too attractive." (or "I/my fault" if よしのん is the speaker? I believe you said that パペット refers to よしのん, yes? Anyhow, it doesn't change the grammatical interpretation of the sentence.)
You can, of course, use せい to express a similar meaning, but you'd have to reword it along the lines of:
...in order to make it grammatically correct.
(In case it's not immediately clear to you why this is, it's because せい is a noun, while いけない is a negative verb, so they connect to the surrounding words like 魅力的 and the sentence final explanatory ～の in different ways. いけない could conceivably also be replaced directly with 悪い for a similar meaning.)