I did a search on Mumsnet for threads containing this name and the only bad things people mentioned were the Cornish comedian and that it's a bit redneck/hillbilly.
I think most people of my parents generation (I'm 29) would make the comedian association but I don't think people my age and certainly my childrens age would think of him straight away. I also think people would have said Seth was too redneck/hillbilly/farmer a few years ago and now it's quite popular so obviously I'm just cooler than you! 
So any other thoughts about Jethro? Will it become trendy do to it's 'O' ending? Like Milo, Otto, Arlo etc?