On AES, is a true random string enough for a key? The string (32 ASCII characters) is entered by a human, and considered truly random. Should a key derive from this string, or it is secure to use this string as a key directly? In other words, does deriving a key from this string makes the encryption more secure?
Even if the 32 characters are completely random, they won't contain non-printable characters. Actually, there are only about 107 printable characters in ASCII (out of 256 values for a full byte) and that even includes the space character. So if all the printable characters are used, it would result to a security level of about $log_2(107^{32}) = 215$ bits, which is quite a lot.

AES reaches the highest level of security for random keys, so in general it is better to put the value through a KDF (Key Derivation Function) together with a salt, possibly a label (OtherInfo) to derive a specific key and possibly - for a Password Based Key Derivation Function such as PBKDF2 or Argon2 - a work factor. That way the security of the key may get closer to the 256 bit strength that e.g. AES-256 promises.

So yes, deriving a key would be more secure. Doubly so since your first sentence "The string (32 ASCII characters) is entered by a human, and considered truly random" is usually not correct in practice. Humans are horrible with regards to entering truly random numbers, even if just because of the typo's when entering it from another screen.