Ditto can take on the appearance, stats, and moveset of any Pokemon. This being said, besides priority moves, Defense, Special Defense, Attack, Special Attack do not matter. Who cares if they're low? Unless you're running Struggle Ditto. The slowness can be remedied by a Choice Scarf. Thus, only HP matters.
If Ditto was fast, no Pokemon would be able to stop it from transforming into another. Good Defensive stats? That means a harder time taking it out before a Transform. GameFreak made it that way so Ditto would not be overpowered.