The minion mitigation passive does do a lot to enable Fizz to be constant aggro mostly based on enemy positioning alone.
Still when I play Fizz I knowingly use my E to drop minion aggro mid trade or as I retreat. So you could say even without Fizz passive Fizz has a "tool" against minion aggro. Which suggests maybe his current passive is outdated.
Traditionally AP Fizz puts all points into E and relies on it for damage. This is because of the high damage upgrade in addition to the huge cooldown reduction and waveclear. The thing I go with 3 points in W at 5 because it is quiet strong for fighting as well, and enables me to use E for utility (like projectile dodge/escape/aggro reset) in early trades and be less mana dependent. When doing so E is still maxed @ level 10.
Minion aggro is not totally disregardable for Fizz though. I mean at least I have to E to reset like when I W + Q, auto, auto then E to safety (my bread and butter trade)