Picture a sine wave with a BIG amplitude. If this is your girlfirend's "mood swings", from high to low, without any apparent reason, RUN. Run away, fast. Doesn't matter if she is smoking hot, beat feet.
A woman like this will crumple to uselessness at normal stressful life triggers. When she's "up", it's great. When not, you will spend 90% of your time trying to get her back on the upside.
Now, everyone has mood swings. Look for a woman who has a much lower amplitude, like this one:
It's OK to get angry or sad occasionally, but you want her to have a decent baseline and not deviate too much. And all this applies to men, to all people, not just women.
Finally, remember that hotness always fades. But a brain is forever. When you have no hair and a gut, when she is wrinkled, you will still have a companion you can talk to, love, and experience life with.