Which is more important: IP reputation or domain reputation?

Domain reputation is more important in modern filtering systems. IP reputation still matters but it can be replaced or reset easily. Domains travel everywhere you go.

Mailbox providers trust the domain more because it represents long term identity. The rise of IPv6 changed everything. A single sender can control an enormous pool of IP addresses which makes IP based reputation far less meaningful. Providers therefore rely heavily on the authenticated DMARC domain since it is the only stable constant.

Your domain is your house flag. Your IP is the temporary ship carrying it.