Yes, “premier cinquième” means top 20%. And it sounds completely fluent in french !

“Premier cinquième” sounds fluent in French, and means top 20%. This is actually used for fractions, so you just have to translate it as a fraction to better understand the value : 1/5 = 0.2 = 20%.
However you won’t hear “deuxième cinquième, troisième cinquième …” very often.

If you want to use fractions, there are a few exceptions :
1/2 -> “un demi”
1/3 -> “un tiers”
1/4 -> “un quart”

For other values, you just have to say the numerator (Top part of the fraction) first as a classic number and then the denominator (Bottom part of the fraction) as an ordinal number (First, second …).

Here are a few examples :
2/5 -> “deux cinquièmes”
5/8 -> “cinq huitièmes ”
10/11 -> “dix onzièmes”

Fraction expressed this way are often seen in written documents. The exceptions that I mentioned “un demi”, “un tiers”, “un quart”, are used orally and most of the time people resort to percentage for further fraction like 1/5, 1/6 …

Hope this helps.

Premier cinquième => Top 20% (to define a position/ranking),

Un cinquième => 1/5 (to qualify an amount)