In Sweden (where Mullvad has its origin) IPSs are forced to keep data on its users, see Datalagringsdirektivet. It does not apply to VPN providers.

Mullvad's entire business is based around privacy, so they have a strong incentive to not collect your data. Your ISP does not have that incentive

If the third party security audits aren’t convincing, then you shouldn’t. That’s your choice to make.