It bears pointing out that she's not apologizing for laughing at an insult directed at Trump.
Just the specific word.
It's quite possible she understood that he was insulting Trump, and the specific word just didn't get consciously registered at the time, until it got brought up and she could pay more attention. So the apology's fine, IMO.
She's still in agreement that Trump's a chucklefuck, and she'd probably laugh if I yelled that out at a rally, no apologies needed.
The apology's aimed at people with learning disabilities and the like, because it's unfair to equate them with Trump's willful shitheadedness.