Being a ham I should joke "Aw, ain't that nice? Puttin the wittle ol' CB antenna on the front, make it feel special! Bless it's little heart!" But I wont. Besides, I also have a CB.
Joking aside, no the front (or rear) bumper ain't ideal, but any proper antenna mounted outside the vehicle is going to work well enough for most of us on the trail. The extra inch or two or three you gain in antenna height won't make squat difference, and the person behind you will hear you just fine, unless they're WAAAy behind you. I have my ham ant. on the rear of my truck facing a "blind spot" to the front. No problems worth worrying about.
If really you want to improve reception, use the biggest CB antenna you can stand to put on your truck. Resonant size does matter for longer distance comm, and CB frequencies (27mhz) inherently require pretty danged big antennas -- ideally over 20 feet! (just ask the DX cb'ers who talk cross country... using BIG antennas)
So the small ones we all use are choking off quite a bit of signal. But who wants to look like a 1970's CB geek with the monster fiberglass whip CB antenna looped over the entire car, with the tennis ball protector on the base, and the tip tied off to the opposite bumper?