The sad thing is, the kind of peaked-in-highschool voter that voted for Trump and can be swung absolutely will care about how he smells. I can hear it now, "I'm not voting for some stinky old man" "I thought the diapers thing was a myth"
This kind of low blow absolutely will have effect if they can make it stick. A sad day for democracy? Definitely. But that's the media environment of today.
Also it gives opponents a chance to needle dick him in debates which will cause him to fly into a rage and embarrass himself. God only knows what TMI he might rave about, his bathroom habits etc. Even tiny jabs like "do I have to be the guy to sit next to Trump" have huge rage potential.