For bad credit, the formal business-like is αναξιόχρεος· for business entities one can also use αναξιόπιστος. Here's also some colloquial ones for bad credit:
I have bad credit = είμαι μαύρος, είμαι μαυρισμένος, με μαυρίσανε· very bad credit = κατάμαυρος· there're some additional expressions related to specifically having lots of debts etc.
Regarding the good credit, one can formally say αξιόχρεος. On a much more colloquial note, one could use λίρα εκατό.