Yes, Mastodon servers can block Nostr content from their server (their users would not see it). They can also prevent their content from being fetched by Mostr, preventing Nostr users from seeing it (although this is less common). This is the same as Mastodon to Mastodon federation. From ​their perspective, Mostr is just another Mastodon server. So this doesn't change whether it's a 2-way bridge. That would be like saying that any individual Mastodon server is only a 1-way bridge to other Mastodon servers that block receiving content from it.