I haven't actually seen this show, but doesn't this ship sorta kinda count as incest, what with Akane being Rikka's creator and all? That'd explain why Taiyaki likes this ship so much, at least.
The show keeps it pretty ambiguous, but the two main theories are that:-
a) Rikka was someone Akane knew and had a crush on in the real world, but she couldn't bring herself to confess to her for fear of revealing her otaku tendencies, which is why she creates a world where she's perfect and popular. Rikka still exists as a character in this world and Akane still pines for her, but can't close the distance. Rikka's character arc is about how she comes to terms with her existence and learns to accept Akane, reassuring her that it's okay to go to the real world and openly express her interests, prompting Akane to leave the artificial, wish-fulfillment world behind; and-
b) Rikka is a part of Akane, representing the 'normal' persona she wore in order to hide her otaku tendencies. The stress of this performance was so great that Akane dissociated from herself, turning the two into separate people. Akane and Rikka's reconciliation in the show represents self-acceptance, as Akane realizes that she can let Rikka be her own person in the artificial world while she tries to live more genuinely in the real world.
All in all, the ship occupies a weird space between incest (at least if you consider a god dating her creation as such), selfcest, and just a standard-ass lesbian schoolgirl relationship, depending on what you choose to perceive Rikka as. Personally, I like to see her as a real person who Akane drew inspiration from to create Artificial!Rikka, because Akane's character arc is complicated enough without multiple-personality shennanigans, and also because the entire point of Rikka's arc is that she's a 'normie' who gradually comes to understand and appreciate the world of tokusatsu by engaging with the people within it. My headcanon is that after Akane goes back to the real world, she confesses her feelings to Rikka and then they begin a genuine, honest relationship as equals.