What the Mastodon's developers are doing – publicly denouncing instances and trying to enforce their moral rules on others – goes against the very core of a free and open internet.
Despite flying under the banner of decentralization and openness, the core developers at Mastodon sure are trying very hard to centralize control over the network. Their attempt at forcing every instance to block those that they deem are not proper, is by definition an attempt at centralization of control. All they need to do to complete the circle back is to implement a centralized authentication system for the protocol.
Sure, they might not have restricted access to the base protocol that Mastodon et al are working under – something which they couldn't if even they tried – but that doesn't mean it's any less worrisome that Mastodon's core developers are talking about hard-coding blocks against instances like Gab. Think what you will about Gab and their dedication to the first amendment, but even considering doing something as absurd like that is, to say the least, the idea of the mentally challenged.
Labeling instances as "problematic" for simply refusing to arbitrarily block certain domains, like Gab, goes beyond being stupid. It's guilt by association, but taken to its extreme by employing thought crime and precognitive-style judgement.
At the end of the day, the developers at Mastodon are about as stupid as the developers at Tor. Core members of the Tor project publicly denounced an individual who went on Reddit to criticize a female Mozilla employee. They went as far as to say that they wished they were found and charged, that the Tor network wasn't intended to be used by people like them. Sure, simply stating their thoughts is quite alright, so long as they do not try and hard-code a way to expose people they do not like, but that doesn't make a statement like that any less idiotic.