Speaking as the admin of a fellow OpenBSD powered instance, it may be a resource thing.
Toots and boosts (especially when boosted by other instances) can put quite a strain on the resources. We are running with eight (x64) cores and 64G of mem.
I think it ought to be rather impossible to use Mastodon through a VMM-powered OpenBSD guest - even for a single user instance. Or that person has to follow just a handful of accounts.