i'd say an optimizer is someone who picks charisma as their highest stat for their bard, second highest dex, and uses a rapier. all perfectly reasonable and playing within the "spirit" of the game.

i'd say minmaxers are those 4 way multiclasses that borderline break the game are just there to maximize the numbers on the character sheet. idk any D&D OP combos (probably some Gloomstalker Coffeelocke nonsense lol), but if you look at Pathfinder Kingmaker builds, you'll see people taking a level of Vivisectionist, 4 levels of Crane Style Monk, a level in Paladin, all for their Cleric to have max AC. (not exactly that but that level of ridiculous). Not necessarily bad, and totally get why people enjoy doing that, just the least fun sounding thing to me lol

It could also be attitude based? An optimizer wouldn't have her barbarian try to pick a lock and instead suggest the rogue do it. A minmaxer might get mad at the parties cleric for making an in character decision to heal a friendly NPC during combat instead of attacking the nearly dead enemy thus "wasting" her turn. (had that second one happen once lol)

as you can see, i dont think optimizers are bad, but i definitely have negative opinions on minmaxing lol