Learn Polish: Jej naiwność to problem. - Her naivety is a problem.

It comes from the adjective naiwny (naive). To form the abstract noun, you simply drop the adjective ending -y and add the suffix -ość. All abstract nouns created with this suffix in Polish are feminine.

Why does this sentence use to instead of jest?

When equating two nouns (naivety = a problem), Polish frequently uses to as a linking word instead of the verb być (to be). The key rule here is that when you use to, both nouns stay in the basic Nominative case. You could absolutely say Jej naiwność jest problemem, but that requires changing the second noun into the Instrumental case. The to structure is very common and often feels more direct.

Since naiwność is feminine, does it decline like standard feminine nouns that end in -a?

No, because it ends in a consonant rather than a vowel, it belongs to a different declension pattern. As a handy shortcut for -ość nouns, remember that their Accusative form is identical to their Nominative form. If this sentence were the object of an action, such as 'I understand her naivety', the word wouldn't change at all: Rozumiem jej naiwność.