Inflation is either good or bad, depending on the circumstances. Same with deflation.
A currency is important only because it is a medium of exchange. A currency is actually working well if it is slowly decreasing in value, because if it does not then people will hold onto it and not use it, thus making it a commodity. In the longer term, what happens is that people will switch to another method to transfer value, and your currency then becomes worthless as the reason for it storing value is removed.
This is one of the reasons why a deflationary by design cryptocurrency is so dangerous. It seems intuitive in our current thinking to increase value by design, but actually this can have dusasterous consequences. Japan, for instance, has had massive problems with deflation and eventually had to work to counter this for their economic health.
Obviously rampant inflation is dreadful, but that’s not what we are talking about here. Low underlying inflation is, somewhat ironically, an excellent indicator of a healthy economy.