Honestly, because Trump does a good enough job of making himself look bad.
Conversely, some people tend to stick up for someone they view as being picked on or bullied. Or maybe they were lukewarm for Trump, but Hillary’s attacks were enough to make them wash off the Cheetos powder on their hands and go vote that day.
If I were her I’d make myself as appealing as possible. Any mud slinging Trump does she can honestly say he is just a big bully and she wouldn’t resort to such underhanded memes. (pun intended)