Yes, trendy is much worse.
I define trendy as "here-today-gone-tomorrow." I do not include classic names that are popular. Trendy, to me, is shallow and near-sighted.
Dated, to me, is a classic name that is no longer popular. These names are, for the most part, classics. Sometimes, for me, the fact that a name is dated actually adds to its appeal. I love the juxtaposition of an "old lady" name on a young child. It reminds me that the 80 year old
Violet was once a baby and it makes me smile.
So yes, trendy is much worse than dated.