pushover

coward, milquetoast, mouse, nebbish, nervous Nellie (or nervous Nelly)

sheep

cat's-paw, mark, target, victim

butt, derision, laughingstock, mock, mockery

booby, dodo, fool, goose, half-wit, jackass, lunatic, monkey, nincompoop, ninny, nitwit, simp, simpleton, turkey, yo-yo

loser

blockhead, cretin, dolt, dope, dumbbell, dummy, dunce, idiot, imbecile, moron

no-brainer, nothing

sitting duck

gimme, laugher, walkaway