It's not. If you're trying to say it can be used as a unit of account, well.. okay, but same for the dollar. But it's inflationary on top of the obvious speculative swings, versus eg Bitcoin that's got a capped supplY.
I'm not sure what you mean by "it's not". Are you saying that ethereum was not twice as valuable a month ago? Or something else? I don't know how this would be connected with whether it can be used as a unit of account—obviously ethereum is only really used as a unit of account for ethereum contracts, due to its volatility.
EDIT: misread! Obviously ETH lost a lot of value.