Pokemon Go doesn't offer any real "pay to win" options like her other freemium games of choice. Sure, you can pay for more resources but you still have to actually go out and play the game. If I had to guess, she probably hasn't dropped much money into it at all and has gotten bored with it since she can't drop a few hundred dollars to obtain a team of super strong, rare Pokemon that allow her to sweep the gyms of Boston.
It would also explain why she's suddenly talking about the main Pokemon games more. While those don't have pay to win options either, Pokemon generators are a thing and not that hard to use. Brianna's attempts at being a "cute, quirky gamer grrrrl" have implied that she's using these programs. Brianna's ego is so fragile that she has to cheat to play a video game for children.