The Apostle is speaking about sound and unsound notions. Since evil conversations corrupt notions, one might expect him to say that evil conversations corrupt minds, expelling truth from them and sowing falsehood.
Instead he says, "Evil conversations corrupt good morals," that is, good habits, character, conduct, and a good life.
An established character cannot be disordered unless notions are disordered. Evil conversations truly corrupt the mind directly. But when the mind is corrupted, by what will you support character? It will immediately begin to be spoiled.
The Apostle passes over this intermediate link, the corruption of the mind, and points directly to the greater danger, the corruption of character. Good character is both more evident and more precious.
If the Apostle had said, "Evil conversations will spoil your notions," someone could have said or thought, "How will they spoil them? They came in one ear and went out the other." Yet that never really happens.
But when he said that they spoil character, he forced everyone to reflect. By this word alone he prepared them to fear evil conversations and evil people.
Or by good morals he understands also frivolous souls, readily inclined to every wind of doctrine, as St. Chrysostom, Theodoret, and others say.
"By these words, on the one hand, he reproaches the Corinthians as unreasonable and easily carried away. On the other, as far as possible, he excuses them for their former errors, removing the greater part of the blame from them and placing it on others, so that in this way also he may lead them to repentance," that is, to abandon error (St. Chrysostom).