In my opinion, system and surrounding talent can make or break a QB, but I'd probably put surrounding talent slightly ahead of system. Even Mahomes saw a noticeable drop in production despite staying in the same system. Same thing with Goff and Stafford when their Olines fell apart due to injury.
Simpson has a strong supporting cast, great coaching, and a QB-friendly scheme with defined reads and progressions. That said, his accuracy, quick feet, and release are traits that can't really be taught.
I think Mendoza could likely put up similar preseason numbers in this environment, but that's just my view from the outside. It'll be interesting to see how both players develop once they're facing live regular-season defenses. Assuming the Raiders get some Oline help.
To me, the biggest factors for QB success are accuracy, processing speed, and the talent around them.