It is stupid for even more reasons than that. I've thought about it back when North Korea had pretend nuclear suicide bombers march on their military parade.

Even if somehow they manage to cram one in a backpack that is carriable by a person, there is no advantage whatsoever given the blast radius. You will have to use a vehicle of sorts to get near the enemy and more importantly away from your friendlies anyway. Whatever distance you can cover on foot afterwards won't make much of a difference. Might as well make it vehicle borne.

Also the whole point of a suicide vest is to be able to get in the middle of an unaware crowd or near your target before using it. With a nuke you don't need to be in the middle of a targeted crowd or get that close to your target to have an effect.