Why are the DD Girls named such?
Their name in katakana: [ディーディー・ガールズ], has no meaning, other than just being "DD Girls"...
How do we even know that they're called the DD Girls?
The DD Girls only appear in the original anime; they are not in the Sailor Moon manga or consequently, Sailor Moon Crystal. In their first appearance in the anime (in the first half of ep. 45), they identify themselves as the DD Girls:
Queen Beryl: In moments, Queen Metalia will be fully resurrected. I will allow no one to interfere. Anyone who can finish these girls [the Sailor Senshi] off, step forward.
DD Girls: Please let us, the DD girls, take on the task.
(For reference, the line spoken by the DD girls sounds roughly like その役目、我がデイーデイー・ガールズに, but my Japanese skills are extremely lacking and I may well be wrong.)
There does not appear to be an official explicit explanation for their name; if there were, I would have expected someone to point it out already somewhere. However, if I had to take a guess, I would suggest that it is simply a matter of themed naming: consider that the DD Girls show up when the Sailor Senshi are at D Point. (They also work for the Dark Kingdom, but the correspondence between the letters is less overt there, so that seems less likely.)
The "D.D. Girls" (ディーディー・ガールズ) were rumored to be based on a very popular J-Pop girl band at the time of the first season being written/planned, known as: "C.C. Girls" (シーシー・ガールズ) (previously known as "D.D. Gaps").
Quite interesting how similar the names are. Both are written in Katakana also. Not sure if it was ever explained officially, but it's an interesting theory.