You are correct in a very technical sense: numbers are not modifiable because they are absolute concepts. It makes no sense to say
Three is purple
because "three" is a mental concept rather than a physical thing that can have color. (Note, however, the distinction between "number" and "numeral.")
However, this is too technical. We can compare numbers and express opinions about numerical facts.
A literacy ratio of one fourth is not acceptable
What your sentence means is that the social fact that such a low ratio of young adults can read is "incredible" in the colloquial sense that it is highly unexpected and therefore difficult to believe.
Would I be surprised if someone in casual conversation used the sentence you first mentioned? Not at all, and it would be understood by native speakers without difficulty. Would it be acceptable to the New York Times in an op-ed piece? Probably not: a good editor would reword and reorganize the sentence. For example,
The approximate ratio of young adults who can read in place A is a disturbingly low one fourth.
Notice that the adverb "disturbingly" conveys a more specific attitude than does "incredibly" and that the adjective "low," which is definitely applied to a number, is intended in a comparative sense to distinguish the actual ratio from an acceptable ratio.