Yeah you'd definitely get better results with a different mic. SM57s are great, but only on snare :lol: Nah, they are the "guitar amp" mic for a reason, but I always find that they sound very toppy on their own (and if you look at the frequency response graph of the microphone you'll see why).
I always find i prefer a warmer, more natural tone for guitar, and therefore tend to use warmer, more natural sounding microphones like the AKG C414 or those T.Bone ribbons I was telling you about. The SM57 mixed with them is great because you can add just the right amount of that classic '57 push. I've also had good results mixing the SM57 with a Sennheiser MD421. :)