I have to agree with the poor treatment of the Tyrells. That being said they may have left some things off screen such as the impact of Tarly switching sides. He has been loyal to the Tyrells for a long time and quite the badass in battle (from Robert's Rebellion and other comments throughout the books). Him switching sides may have convinced over half of the other lords that the time had come to switch too. The Reach really dislikes Dorne, they were on opposite sides of Robert's Rebellion and Oberyn crippled their eldest heir (not cast in HBO land) so with Dorne allying against Cersei I could see the appeal for some of the southern Reach lords to back the play against Olenna. Who also has a caustic demeanor and a woman which may have also weakened her standing.
I mean its a bit of a stretch considering anyone with half a brain could tell that Cersei murdered Lord Tyrell, the septons, Loras and much of the court that was attending. This really all feels like they want to play up the danger and power of Cersei to make Dany's victory seem more meaningful than what should actually happen which is an outright trouncing. Cersei is a drunk, vicious evil idiot. She makes bad decision after bad decision and the show writers decided to go the complete opposite the direction with her, somehow making her Tywinn Jr. when she was not at all like him.