It is nominal. There is only one mic (ie one side) and if you record in stereo mode by default the left channel is selected for this alone mic. To record in stereo you need 2 mics with 2 separate entries on your sound card.
Try to select mono in the record submenu (item 3, "advanced")
I assume as you are recording via line in that the guitar is an electric and not acoustic.
Unless your electric guitar has a stereo pickup and the preamp is also stereo then you will as CBY TLSE has said only get mono on the left channel.
There is a trick though - in MMM or Music Studio you can duplicate the left channel into the right and then using the mixer adjust the stage position as heard in stereo.