I hesitated to reply because I've not used fiddle~ in ages, and don't know a whole lot about it. I had the same experience with guitar of not being able to detect below about G#.
Have you tried the sigmund~ object? If I'm not mistaken it's a newer "version" of fiddle~ from Miller Puckette. It may have improved low frequency pitch-tracking. There is also the pitch~ external, similar to both but I have no idea whether it will respond differently to low frequencies.
One thing to try could be pitch-shifting your guitar signal up by an octave (gizmo~) before the input to fiddle~. It might make the tracking go a little mental, I'm not sure. But if that worked well enough, you could just use the pitched-up signal to detect the notes you're playing and then process your dry guitar signal normally.