There's nothing wrong with speculating using other people's money as long as you're honest about it. Most mutual funds operate that way.

Do mutual fund investors accept that their money is on the line? As the saying goes, a financial crisis is when someone loses money and it's socially unacceptable for them to not be paid back. So perhaps a bubble is when you speculate with money that it would not be socially acceptable to lose.

Yes mutual fund investors accept that their money is on the line. Many mutual funds have negative returns. Your definition of a bubble makes no sense.