The loan is written for the persons applying. The home is only used as collateral. Since your name is not on the loan application, your credit (or lack of it) will not have any effect. You should however be required to sign a notice stating that you acknowledge that a loan against the property has been approved.
All applicants on the loan come into play. Everyone’s credit is reviewed. A good credit applicant can not be used to offset a bad credit applicant – even on a refinance.
If the bad credit person is NOT on the loan application, then there are no issues.