Nah, Simmons isn't averaging 20ppg without Embiid out there to draw defenders or space the floor for him, what Mitchell is doing is every bit as impressive if not more so than Simmons. The Jazz are asking Mitchell to be their number 1 option and he's doing it as a rookie with very high efficiency, there's no way Simmons would be able to score 20ppg efficiently and helping the Jazz win, he has too many flaws on offense to do that.
The Sixers live and die with Embiid, they are 25-17 with him in the lineup, Simmons has been good but the last 2 months for Mitchell has been better, especially when you consider that he has been the man for the Jazz, while Simmons has been the sidekick to Embiid.