I'm not an expert on economics or crypto, but I think depending on your definition you could say Bitcoin is a bubble if it does not reach and stay around at least, say, 12k in the next 6 months.
The reason being that Bitcoin reached 20k around 6-7 months ago (and was last at around 12k 3 months ago, but obviously it has dropped off a lot since then). So if it cannot sustain at near 50-60% of that peak within a year from said peak, than you might call it a bubble.
Right now it seems to be settling at 7 to 8k, which has obviously hurt a lot people. But with something like crypto, you never know. It's different from most things we've seen before A year or two from now it could be worth 30 or 40k then nobody would be talking about the "Bitcoin bubble".