The thing that some people have an issue with I suppose is she got parachuted into a top level job in the RPG world with no D&D design credits. There are posters here on ENworld with more published work than her.
The women Paizo hired more or less done the same thing Mearls did. They worked for 3pp/did freelance work, got published, got noticed, got more work, eventually got hired. Amanda and Crystal did that, I was looking through old Dragons and found an interview with Mearls in 2000 Dragon d20 special. Mearls also had a few adventures published in Dungeon for 3.5.
The new hire has skipped that process which I think is what a few people would expect, she has not done her dues in the trenches so to speak. Its not unprecedented in the D&D world but you would probably have to go back to TSR to find examples as most of the 3E and 4E crew were either from TSR or had been published in things like Dragon and Dungeon. With no Dragon and Dungeon you would expect published 3pp and freelance work and the DMGuild.
IDK if its unique in the modern D&D era, but it is unusual to say the least hence why I think some people are accusing her of being unqualified. She has skipped a few steps most designers have gone down. Getting published and getting noticed has been the more traditional way of doing things I suppose nothing to do with her gender although it looks like some idiots have made it an issue.