If you're talking about "The Competitor" achievement, it's complete 200 matches online OR offline.
I don't see any other achievements that involve 200 matches so I guess that answers your question.
It's definitely online or offline. I completed all of the 'cheevos a lot faster than I expected to. Try to do runs through UMK3 though. The matches are way faster with combos.
Considering that I don't have an XBL gold account and have only played against the AI so far, I was pretty damn surprised when I unlocked this achievement. It didn't even take me that long.
I'd say about 20 of my 200 matches came from online. When MK:AK came out, I dropped MK9 for awhile because of it lol.
The only thing that can make me quit MK is another MK game. UMK3 expert ladder all day, kid. Robo Smoke 8)