Yes, a ciphertext of a bulk encryption algorithm normally should not be compressible to less than the plaintext size¹ (at least, if the compression function does not know the encryption key), other than in some corner cases which will occur only with negligible probability (like you hitting the one plaintext which will encrypt to the all-zero-string).
¹Of course, if the encryption algorithm applies some encoding after encrypting which makes the ciphertext larger, like PGP's ASCII-armor, Base64 or Hexadecimal encoding, you can compress this resulting encoded ciphertext, but not to less than the original plaintext size. Similar things could occur with some asymmetrical encryption algorithms, which have slightly biased output, when (mis)used for bulk encryption - don't do this.
If the compressor knows the algorithm and key, it can simply decrypt the ciphertext and then compress the plaintext, of course.
With bad modes of operation, such as ECB mode, some repetitions in the plain text will also show through in the cipher text (see this famous example in Wikipeda), which then might show by compressibility of the ciphertext (the ECB image is bigger than the plain"text" one, but still smaller than the CBC one).
On the other hand, non-compressibility of ciphertext with generic compression algorithms is given by every not totally trivial encryption algorithm, even if they have to be categorized as broken by every aspect.