I don't think it's fair to describe the league as NA. While the developer is American and the league will certainly be NA-centric, everything that Blizzard has told us about the Overwatch League so far is that they envision a truly global league. The opening matches will be played in America and the majority of the teams are American (although they may shoot to make that only a plurality down the road, who knows!), but I believe Blizzard hopes to cater the league to an international audience. There is a lot of money that can be made from attracting the eyes of a nation as large as China and from a nation as esports hungry as Korea.
I do think you're right that it isn't as appealing to a casual viewer for a Korean on Korean matchup to happen, however. I also think it's equally detrimental if a team seems to dominate a league in its infancy, too. While APEX isn't the ideal conditions for a visiting Western team and we don't know how Rogue and EnVyUs matchup to the top of Korea on a more level playing field, we do know for certain that Kongdoo Panthera is capable of defeating Lunatic-Hai and their matches are almost always close. I do think it's important to have Kongdoo Panthera representing someone in the league to ensure that there is a competitive final (not saying a western team doesn't exist or can't be built to compete with LH, I am just saying that KDP is closer to a concrete guarantee of being a competitor to them).
In this sense, KR vs KR isn't too bad. We know that there will be rosters that will be mostly western and all-western existing in the first season of OWL. It is very likely Seagull will be on one of these rosters, and a lot more casual fans will watch when he does play like they do now. I imagine seeing a team defeat his, and that team's players specifically outplaying him, will pique the interest in a lot of those fans before a match between those two Korean teams ever takes place. I might just be overly optimistic, but I do believe that once fans are exposed to watching insane players play at a high skill level, they will be more interested in watching those players regardless of nationality. Although I might be reiterating what KuroiRyuu already said. If I am, I want to make it clear that I don't mean the OWL should be made up entirely of all-Korean rosters, anything more than 4 would definitely not look great and even that number is pushing it. Two to three is alright.
While I think owners will end up bringing more than just two all-Korean teams in their efforts to win right off the bat, as C9 looking for six Korean players seems to suggest, I don't want to see the league dominated by Koreans. Just as much as having only American franchises right off the bat would go against the vision of a global league, having one nationality dominating the playerbase of the league does the same thing. While I think it's not as detrimental in the first year as it would be once the teams actually move out of Los Angeles, I still think it has some detriment. Like Sideshow said, building a team off of Korean free agents would be really difficult, and if C9 does end up bringing in 6 Koreans to play for their team, I hope they struggle. Not out of malice, but as a way to show Koreans are beatable to casual fans.
In an ideal world, I think rosters composed in a similar way to that of Immortals or EnVyUs is best. Mixed nationality teams seem like they should be the natural conclusion for owners when it's a global league taking from the global playerbase. The teams should be trying to create chemistry with the best talent from around the globe. Of course language barriers get in the way of this, but hopefully that becomes less of a factor for eastern players that may spend a large amount of time in America for the opening seasons of the Overwatch League.