No. Calling someone a coward is not ad-hominem, it's just an insult.ad ho·mi·nem
/ˌad ˈhämənəm/
adverb & adjective
- (of an argument or reaction) directed against a person rather than the position they are maintaining.
An ad-hominem is "your argument or conclusion is wrong because you put it forth."
Examples abound, unfortunately: in an experiment recently conducted, when a sample of college students was told that a planned action was Obama's proposal a majority was in favor of it, but when another sample was told the same planned action was Trump's proposal, the majority was rabidly opposed to it.
And this is college students. Supposedly smart, but not so much.