My first suspect wouldn't be the sm57, I've recorded with those without any problem straight into the computer.
The Tascam US 122 usb interface settings is where I'd start. It should have something like a control panel or a mixer that you can open and then adjust the input levels.
I just googled Tascam forums and found this:
TASCAM Forums Member Supported
Post the question there, those folks should be able to give you very specific answers to your question.