The adverb decidedly is often a highfalutin word for certainly, clearly, or very. And like those adverbs, decidedly can be useful in specific circumstances—especially where it means emphatically or resolutely—but more often it could be removed with no loss of meaning.
For example, would any of these sentences be less clear without decidedly?
The pieces premiering in Milan introduced shapely curves and organic materials for decidedly softer, warmer looks. [Los Angeles Times]
The greens were firmer and some of the pin positions were decidedly snotty. [Independent (article now offline)]
But in recent years, the movement has betrayed a decidedly more shrill and cultish character. [National Post (article now offline)]
In the second and third examples, decidedly is a hedge word that distances the writers from harsh adjectives, allowing the writers to use the harsh words without taking ownership of them. In this use, decidedly is just a more respectable-sounding version of pretty, somewhat, and sort of, which are likewise questionable when used as hedge words.
Decidedly makes more sense as a synonym of emphatically or resolutely—for example:
Soon after my arrival here, I am met by an Egyptian friend … who is Muslim by birth but decidedly secular by choice. [Wall Street Journal]
Thus we get the “non-partisan” party, which is decidedly not non-partisan. [comment on Vancouver Sun]