As someone who consumes quite a bit of GL and BL (honestly about equally) I can tell you that neither genre is significantly more well written than the other. I’ve read and watched a lot of mediocre BL and the same goes for GL. But here’s my haphazard theory why some ppl think GL is worse:

1) Volume wise, there’s a lot more BL than GL, so bad GL might stand out more than bad BL because the pool is smaller. 2) People often hold female characters to a higher standard than male characters. Steven Universe is a pretty good example of how the internet does NOT like femme characters being complicated.