Depends what you mean by 'trendy'? Old man chic is definitely a big current trend. But I wouldn't describe Frank, as an individual name, as 'trendy'.
Trendy to me seems more modern. If you asked me what boys names are currently 'trendy', I would say things like Roman, Rowan, Arlo, Hunter, Hudson, Oakley, Otis, Otto, Jude, Jaxon etc. I'm not sure Frank fits the 'latest cool trend' description, despite growing in popularity due to fitting in to a current trend of using 'old man names' on babies.
Franklin or Frankie would actually seem more 'trendy' than just Frank. I'd lump Frank more with names like Arthur, Albert, Stanley, Alfred, Wilfred, Frederick, in that all are currently on the rise due to fitting an in trend, but are too traditional to feel trendy individually.
I'm probably not making any sense but I know what I mean lol.