Bubble is acceptable in terms of something going way higher in value than it suppose to be, currency could never have that bubble issue because that is called inflation not bubble. For example when Venezuela printed money they had a big trouble and now their currency is a lot worthless then it used to be (not like it used to be worth a lot neither) but that is called inflation and bad inflation could cripple a countries economy for years to come.

Bubble is some product or a stock or something that gets valued way more than what it is IN currency calculations, for example something that costs 10 bucks now valued at 100 tomorrow and 1000 next day is bubble because it probably doesn't worth that but valued at that, 1 dollar will always be 1 dollar, even if purchasing power drops it will still be 1 dollar and that is why it is inflation and not bubble.