Let's clear up a couple of things first...
Only 32 players are seeded in the women's singles draw at the French Open this year. The rest of the field are not seeded, qualifiers, or wild-card entries.
Also - a players "talent level" does not necessarily correspond to their position in the draw or even their seeding. Kristina Mladenovic was in the main draw only because her world ranking 2 weeks prior to the French Open was high enough to warrant her being there. She's currently ranked 103 and has been ranked as high as 36 in her career. It was random that she faced Li Na in the first round - not because she's seeded/ranked #127.
Lastly - don't forget about the "bad day at the office" factor in tennis. As we saw with both Li Na and Serena Williams at the French Open this year - if you have one bad match where you're not playing your best - and your opponent is - you're out. That's the harsh reality of tennis and single elimination tournament competition.
Now, to answer your question...
It sounds like the seeding method you are thinking of is how the Men's NCAA D1 basketball tournament brackets are drawn up (with the #1 seed playing the #16, #2 playing #15 and so on) - but that's not really how tennis does their tournament draws.
In tennis - the seeded players are each put into their own area (usually a quarter) of the draw (how many seeds there are depends on the tournament and the size of the draw). For example, the #1 seed will be in the first/top position of the draw, and the #2 seed will be in the last/bottom position of the draw. This is done so that the #1 and #2 seeds can't possibly face each other until the final match of the tournament.
Once you go past the #1 and #2 seeds, it starts becoming more random about where the seeds fall into the draw rather than a specific formula being applied.
All that is guaranteed after #1 and #2 placement is that the following 6 seeds (#3-#8) end up evenly placed into the 4 quarters of the draw - so again, for the French Open, which is a draw of 128 players for the Women's Singles - two of the top 8 seeds will be within positions 1-32, 33-64, 65-96 and 97-128. Usually those two seeded players go into the first and last positions within those numbers (so they only face each other at the latest possible round).
After that, where the seeded players fall in the draw becomes more random (names drawn from a hat) - and where non-seeded, qualifier and wild-card players fall is also random. The fact that Li Na faced Kristina Mladenovic in the first round had nothing really to do with anything like seeding or ranking - it was just the luck of the draw. I'm guessing that 8 or 9 times out of 10, Li Na wins against a player like Kristina Mladenovic - it just happened to not be her day during that match.
To further show how the draw is more random for non-seeded players, Kristina Mladenovic played Alison Riske in the second round - who's currently ranked #45 in the world - and she also won that match. From what I've seen of lower-ranked players in grand slam events that beat a very high ranked player - the lower-ranked players usually "flame out" in the next round because of their emotional high coming off the "fluke" victory the previous round. Many examples of that come to mind - Stakhovsky beating Federer at Wimbledon and losing in the next round, Lukas Rosol beating Nadal at Wimbledon and then losing in the next round, etc. It's pretty common to see.