Even in casual mode, you have a (hidden) rank which does go down the more you lose. If you keep getting demolished in the game then I'd recommend doing the training, playing against bots, and/or continuing to play in casual mode until your rank starts placing you with other players you can better contend with.

There's also a distinct possibility you were against someone smurfing as some people like to do that either for content or just for kicks and giggles. The very lowest ranks are probably where the most egregious smurfs like to keep their ranks, so unfortunately you can have some of the widest swings in actual skill levels.

Unfortunately the devs don't really seem to care much about the smurfing problem in the game because it's been pretty rampant for a very long time and some of the changes they've made have actually made it even easier to get away with.