JAG: It just means that you have one style. Remember, it's MIXED martial arts. Examples: Maybe you have excellent stand up ability, a great striker, but should you be taken down, your akin to a turtle on it's shell. Another would be vise versa, maybe your a submission specialist, but when both fighters are in a standup position, you really aren't a very good striker so you have to resort to wasting energy in several takedown attempts. That's really all it means.
this basically means that a fighter has\ or tends to lean towards only one style of fighting. if a guy uses nothing but take downs and submissions he'd be a one dimensional grappler as opposed to a fighter how would combine his grappling with striking, hence the term mma(MIXED martial arts). if you look at early ufc (1-8) you'll see every fighter used a different style, boxers just punched, wrestlers just took you down and\or suplexed you, and so on.
A one dimensional fighter is someone who is really good at one discipline, but is not considered well rounded. An example would be an amazing kickboxer who is helpless on his back, or a jui jitsu black belt with zero standup skills, who just drags his opponent to the mat. The opposite of one dimensional would be well rounded.