Not necessarily. 'The market can stay irrational longer than you can stay liquid', etc. To short you not only have to believe something will eventually go down, but that it will go down soon.
which makes sense, because the market is cyclic, there's going to be a crash sooner or later. crying "it's going to crash" doesn't say anything useful.
There's no good way to short BTC today. Real financial institutions hardly touch it, and if you make a bet on a crypto exchange, there's no guarantee you'd ever get paid. If BTC tanks, likely the exchange will go away too, and you won't be able to cash out.
people do, but they often lose money and you never hear from them (except for the rare instances where some make money and then they don't shut-up about it)