This would turn every pvp player into casuals and the highest mythic+ players with 18hours/day to casuals aswell.
I would just call players that only participate in the LFR/WQ/TW - without any evil intention - "noobs".
I would call a casual player someone who min-max in their limited time the game, just as the most hardcore players would do, but since the gametime is limited there are choices to make what you do and what you skip in this game.
There is nothing "casual" about players with 18h+/days farming gold millionaires or the not-raiding but 5000+ dungeon running mythic+ community, dont you think?
Time fits for the game genre much better, since its an 24/7 MMO without a finish line.