As the developer of a lesbian protagonist game: probably.
While I disagree that there's a small market for lesbian content ("lesbian" is consistently a top-3 [2019] if not overall top search term on PornHub and other aggregators), I do think that the primary base for lesbian content mixes enough with hetero content that most of that base views it as a supplementary focus rather than a primary one. For example, you'll see lots of male protagonist games that feature some lesbian content here or there, or even a female protagonist game that is at most a 50/50 split between lesbian and hetero content.
Why? Well, easy: most of the audience for lesbian content is also interested in hetero content and they get more variety out of both. Plus, you'll attract that arguably larger core audience that's NOT interested in lesbian content (who will find a way to bypass that part). I'd further argue that most of the audience that isn't interested in hetero content have gotten good at training themselves to ignore it for lack of alternatives. So you get two otherwise exclusive audiences, plus the combined audience, for the price of one.
From a data standpoint, I think there's also a lack of good comparators since there just aren't many lesbian protagonist games. Maybe Large Battleship is the most successful developer of lesbian-only games to date, but I wouldn't say any of their games have taken off the way any of this site's trending games have. I'd probably argue that the most successful games with a lesbian focus belong to LessonOfPassion and VDateGames (Chaotic) - and there's no Patreon data to those two (though I'm certain both take in more than $200/mo). But, even there, both mix hetero and lesbian content and have much wider selections of games available.
tl;dr - if you're in it for the bucks, boy-girl focus will probably earn more. Even more if you focus on fetishes people seem to lose their minds over (*cough* incest *cough*). I really hope there are some lesbian-exclusive games that start to break out, though, since they're my favorite and I would LOVE to see more.
And on a personal note, money's not involved in my game - so I'mma just keep making the all-girl game I want to play!