He was just pointing out that:
Since Ramsay refused to save lives of his men by refusing single
combat challenge by Jon, why should Bolton men feel loyalty towards
him and want to fight for him?
The complete dialogue was:
Jon: There's no need for a battle. Thousands of men don't need to
die. Only one of us. Let's end this the old way. You against me.
Ramsay: (chuckling) I keep hearing stories about you, bastard. The way
people in the North talk about you, you're the greatest swordsman who
ever walked. Maybe you are that good. Maybe not. I don't know if I'd
beat you. But I know that my army will beat yours. I have 6,000 men.
You have, what, half that? Not even?
Jon: Aye, you have the numbers. Will your men want to fight for you
when they hear you wouldn't fight for them?
Jon challenges Ramsay to decide that in the old way i.e. one on one combat, winner takes Winterfell. Ramsay was at advantage, He was certain of his victory but he was not certain if he could defeat Jon or not. Therefore he decided not to risk everything on that one combat and go for full battle instead.
If Ramsay had accepted, Bolton men would have been able to go home whatever was the outcome of the single combat. But since Ramsay refused, that means he does not want to save lives of his men and values his own skin more than theirs.
Ramsay recognizes Jon's retort of his refusal as a subtle attempt of sowing disunity and resentment among his men and replies:
He is good. He is very good.
Though it appears Jon may have "attempted to sow dissent" unintentionally because he says that his plan was to make him angry (and thus presumably goad him into one-on-one combat in which Jon would have prevailed). Poor lad really knows nothing.:
Tormund: Did you really think that c**t would fight you man-to-man?
Jon: No. But I wanted to make him angry.