Learn German: Der Wert treuer Kunden ist immer wichtig. - The value of loyal customers is always important.

It is describing Kunden (customers), which is in the genitive plural (of loyal customers). Because there is no article in front of it (a zero article), the adjective has to show the case. It takes the strong genitive plural ending -er.

Normally genitive nouns get an -s or -es added to them. Why doesn't Kunden have one?

First, Kunden is plural here, and plural nouns never get an extra -s in the genitive case. Second, even if it were singular, the word der Kunde is a weak masculine noun (n-declension). These nouns take an -n in all cases except the nominative singular, rather than the usual -s.

Could I say Der Wert von treuen Kunden instead?

Yes, using von plus the dative case (von treuen Kunden) is a perfectly correct alternative and is more common in everyday spoken German. However, the genitive version used here (treuer Kunden) sounds more professional and is standard in written German.

Why doesn't the word wichtig get an adjective ending?

Adjectives only get endings when they sit directly in front of the noun they describe. Because wichtig comes after the verb ist to describe the subject, it is a predicate adjective and stays in its base, uninflected form.