It’s not optimism that stocks will only go up, but it’s optimism that they won’t suddenly go up or down by too much, which is generally to be avoided unless you’re a trader.
Right, but “not go up too much” isn’t really the normal definition of optimism when it comes to the market. If it was, an even lower implied volatility on a flat market would be irrational exuberance!
A stock that has gone up like a rocket ship will have a high implied volatility despite no crashes or expectations of declines.