It is a small annoyance of mine, but I wish as a society we could agree exactly how many dimensions characters in fiction should possess. When I was younger, I generally heard "two dimensional" as a compliment, while possessing only one dimension meant a character was flat and stereotypical. However, in more recent times, I am starting to to hear "two dimensional" as an insult, with a character needing a full three dimensions to seem realistic. It would not be a problem, normally, but since the dimension counts overlap, with two being good or bad depending upon the speaker, that it can leave one confused whether something having two dimensions is being praised or insulted.
I can see arguments for both sides.
The original version seems pretty self-explanatory. It is not so much based on the real world measurements, but rather uses "dimensions" as a description of aspects of a character. A character with but a single attribute, having but one dimension, is a flat, lifeless character, while possessing additional attributes, that is having two or more dimensions, makes them more lifelike.
The newer version, like most modern misunderstandings of cliches (eg replacing "rein" with "reign" in "free rein") is the result of failure to understand a word. Seeing "dimension" purely in terms of physical measurements, in terms of length, breadth and depth, it imagines a character must have "three dimensions" to seem lifelike. This is probably reinforced by the tendency to use terms like "flat" to describe characters that are not compelling. And so, it is assumed "two dimensions" would mean unrealistic, while three dimensions would be good.
Personally, I favor the original, as its use of dimensions seems more relevant, but in the long run I don't really care. I just hope we eventually decide as a society to settle on one or the other, as the present mixture of cliches can be quite puzzling. (Given the tendency toward adopting the more stupid version -- eg "I could care less" -- I assume three dimensions will eventually win.)