The adverb decidedly, meaning (1) without doubt or (2) without hesitation, is often a highfalutin word for certainly, clearly, or very. And like those adverbs, decidedly can be useful in specific circumstances, but more often it could be removed with no loss of meaning.
Example
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. In this sense, decidedly is just a more respectable-sounding version of pretty, somewhat, and sort of, which are as questionable as very and clearly.
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]
