Wikis often have false information; I myself don't know what the cataclysm was for sure. I'd ask him over at FUNi's forums for clarification, just to make sure... if the Tenchi wiki is anything like the DB wiki it can't be trusted

Yeah I don't know whether to take this as canon or not, that's why I created this topic.

Something happened to Mihoshi in the past. Exactly what it was I can't say off-hand, but that's why she's never seen without her hair ribbon (It's an inhibitor of some kind. Take it off and bad things happen), and why that image appears. It's also the reason why her performance in the GP went down, and her extremes of luck.

I'll see if I can find out at some point.

From 101 Secrets:

72. Why does Mihoshi never undo her pony-tail?

Answer:
Because if she does, she will lose her memory.

More details ...
She ties her hair in pony-tail even in the bath. This is because she will not remember anything while she is without it. Why? It cannot be revealed now.

At some point, I've also read something similar about the ribbon too, but I thought that it suppressed her memory, so that she couldn't remember her past. I'm not 100% certain on that though. Even Kagato acknowledged Mihoshi's "heroic deeds of the past" so indeed some form of severe debilitation had to have occurred to have her now being "washed up" and her "performance stinks". I've never read anywhere exactly what caused that, but I guess that a botched suicide attempt is just as good an explanation as any other. Having the ribbon suppress memories of that attempt would help prevent Mihoshi from falling back to that frame of mind again, and thus avoiding another suicide try (just me speculating here).

Before OVA 3, Mihoshi seem to be a typical sterotypical dumb blond like in Tenchi Universe and Tenchi in Tokyo. Then we have learn that something happen to Mihoshi before she came to the Earth. Mihoshi's mother Mitoto has the same carefree attitude which Mihoshi has. Also, Mitoto is definatley no dumb blonde. She is an expert on antiques. She may be play the dumb blonde as well as any actress.

Mihoshi's family may ask Washu to find a cure for Mihoshi's current condition. We will suspect the outcome when Wahsu examines Mihoshi's DNA. Washu will find out that Mihoshi is related to her.

I'd just like to point out that some of us did know about Mihoshi's past problems long before OAV3 came out. If you didn't, that's your problem, not the shows.

Personally I doubt it was as simple as a suicide problem. From what I recall it was exposure to a higher-dimensional event, but aside from the image in her brothers flashback (himself someone we've known about for a long time), I have nothing to back it up.