Deflation is "bad" in the way that it postpones consumption.

Imagine you wanted to buy a new TV set and you knew that next month its price would be lower, it would be quite rational for you to postpone your shopping until next month.
But then again, next month, you would knew that the price will drop more if you wait more...

Deflation is good for hoarding, bad for consumption whereas
Inflation is good for consumption, bad for hoarding.