In practice, with any real, widely used compression algorithm, encrypted output will never be compressed. You can generate 100,000 random binary files and run them through all the major compression tools and not one of them will compress by even a single bit.
In theory, some random outputs will be compressible by common tools (for example, a file that's all zeroes). However, the probability of encountering a random file that compresses more than the overhead from any common compression tool is vanishingly small.
In theory, some random outputs will be compressible by common tools (for example, a file that's all zeroes). However, the probability of encountering a random file that compresses more than the overhead from any common compression tool is vanishingly small.