What if my fiance's name is misspelled on her B1/B2 visa or some other document? Will this cause problems when applying for a K

Generally speaking minor discrepancies in documents like the one you describe are not a major problem. In your case it seems that her name is spelled correctly in all other documentation so it should be clear to any examiner that it was simply a clerical error. Additionally, the nature of the B1/B2 visa should not really be an issue in your fiance visa case. In any event, with this situation, as with any other similar situation I think that it is best to deal with the matter head on and point out the discrepancy in the cover letter to the USCIS. I would simply just note the discrepancy and indicate that it was a clerical error. Nothing else should be required. Best of luck.