An "all-time low" is as low as you've ever gotten.
If you are depressed, you can hit an all-time low and never have been more depressed than you are at that moment.
However, it can also refer to behavior. If you do something and someone says "that's low" it means it was a sneaky, mean, underhanded thing to do. So if you've hit an all-time low in that sense, your behavior has never been worse.
An all-time low could refer to morale, behavior, a stock-price, the worst insult ever.. it just means "as bad as it's ever been, to date."