Kbin user here. It does not federate downvotes from lemmy. So far, I have a total of two (2) downvotes and every single interaction, including the one I got downvoted for, was quite positive.
No toxicity in normal interactions so far. The only (slightly) toxic comment sections were regarding meta topics of users complaining about toxicity elsewhere and/or wanting to defederate more communities. Even those discussions were nearly entirely polite and productive.
The only somwhat toxic topic I participated in was when one car-enthusiast complained about the fuckcars community and got called out throughout the comment section. Piling on like that was probably not the best way and they deleted their post some time after.