Truth be told, I never enjoyed Overwatch casual mode. You have slightly more success going doing crazy stuff on Quick Play than on Competitive, but organization (read; healers) will still rule the day.

Even if you want to «play seriously» and try to pick whatever your team needs on casual mode, it will not mater much if your team has 3 hanzos and 2 symmetras.

Casual is where you go to farm boxes, train new heroes and have fun as long as winning isn't part of the equation.