JCL itself has a lot of warts, no denying that. But the basic idea behind the DD statement is interesting. In Unix, file descriptors are just numbers; whereas, in classic MVS, instead of numeric file descriptors the API uses 1-8 character names (DDNAMEs). While in principle a Unix program can inherit any number of file descriptors, in practice very few programs inherit any more than 0-2, because remembering numbers has poor usability. In classic MVS, it is not uncommon for a program to inherit more than 3 file descriptors, because they are assigned names not numbers. ("File descriptors" and "inherit" are not quite the right terms, but trying to use Unix-like language to make it comprehensible to people.)
(And I say classic MVS, because contemporary z/OS is actually a conforming Unix implementation so it supports numeric file descriptors in the Unix API, as well as alphanumeric DDNAMEs in the classic MVS API.)
Unix Programmer: "Huh? Why did you comment out the entire program, except for that one function?"
Mainframe Programmer: What? No, those are JCL statement lines."
Unix Programmer: "..."
A more obvious example of a wart would be how it exposes low-level details of disk layout (e.g. space allocation in cylinders and tracks), which improved performance and simplified implementation back in the 1960s, but which nowadays is just useless complexity–modern disks have completely different physical layout, and you have to buy expensive mainframe-specific SAN controllers which know how to simulate the ECKD physical layout of legacy mainframe disks on top of modern industry standard disks/SSDs.
I guess this happens less often otherwse there would be many more of this kind of joke.