When you say an asset is a "bubble", it means that it would not prosper or last for a long time. It may gain its peak quickly, but after a while it would be doomed and plummet in value similar to how a bubble may abruptly float and then pop. I think that description is not applicable to bitcoin because it has been present for a while now, for about a decade to be exact. And although its value is going up and down at a very quickly, I think its just one of its features because its not backed by any reserves like the traditional currencies, which pumps value on it readily to avoid the effect of inflation.