THE SHORT VERSION IS, LIKE SO MANY THINGS, IT'S BECAUSE OF PORTLAND

she and foster knew each other bc they and amethyst were all werebeasts in the alternate reality thing, plus foster and amethyst were siblings and miko and amethyst knew each other through UNDERGROUND WEREBEAST WRESTLING so foster was like. essentially her cool older friend's cool older brother, whose awful sense of humor she considered to be Great

plus miko is exactly the kind of disaster who thinks that it's really cool how foster makes terrible decisions constantly so it's also just a matter of her standards being VERY BAD