Because duidelik comes after is and is being used predicatively.
In Afrikaans, adjectives used after a linking verb like is usually stay in their basic form:
But when an adjective comes before a noun, it often takes -e:
So:
A rough English-friendly guide is DOY-duh-lik or DUY-duh-lik, but the vowel in dui- does not match English perfectly.
A few helpful points:
You do not need a perfect accent at first; being consistent and clear is more important.
Usually, no.
In this sentence, die helps make telling a specific noun phrase: the tally / the count.
Without die, the sentence would usually sound incomplete or unnatural in ordinary Afrikaans. Afrikaans often needs an article where English also needs one.
So the normal sentence is:
Die telling is duidelik.
Yes, it is grammatically normal and natural.
It has a very straightforward structure and uses common Afrikaans grammar:
The only thing that may depend on context is the noun telling, since its exact naturalness depends on what kind of count/tally is being discussed. But the sentence itself is perfectly well-formed.