What a 0/5 rating means
On the comedogenic scale, a 0 means it will not clog pores for essentially anyone, including very acne-prone skin. That places Ascorbic Acid in the range most people, including many with acne-prone skin, tolerate well.
One thing the number cannot tell you is concentration. Ingredients are listed in descending order, so Ascorbic Acid near the end of a label is present in tiny amounts and matters far less than the same ingredient near the top.
About Ascorbic Acid
It is an active ingredient included for a specific skin benefit. Pure vitamin C, rated 0/5 for clogging. Can sting at high strengths but does not block pores.
On a label it can read as Ascorbic Acid, L-Ascorbic Acid, Vitamin C — worth knowing when you scan an ingredient deck.
Ascorbic Acid in makeup and skincare
It is formulated into targeted treatments and some makeup-skincare hybrids. Its irritancy is rated separately at 0/5, which is low.