Concerning character tierd, consider the fact that magma trooper can beat a traya squad. The whole point of this mode is to strategize how best to use your roster, and then how best to set a defense. If your GP is skewed by fluff, it's up to you to start focusing on gearing certain characters higher, and focus on mods.
There is no gp difference between a purple 5* mod with no speed and one with 15 speed, for instance. It's not matchmaking's fault if you've ignored mods, and matchmaking based on mods would be crazy, considering that's sort of the point from a business perspective.
The fact is that there are a lot of different kinds of players, focused on different areas or styles of the game. If you want to focus on GA, it may take some time to adjust your roster, but that's what you'll have to do. Otherwise, set a defense, and enjoy some free rewards.