At the moment the front page of JoinMastodon is implying that people who want to join Mastodon should join mastodon.social. This is causing the Fediverse to centralise, as the percentage of active Fedi users on mastodon.social has climbed to almost 30% (it used to be about 10-15%). If you look at just Mastodon servers the percentage is even higher.
It is conceivable that mastodon.social could soon reach 50%, and at that point the Fediverse would be centralised as most of it would be under direct control of Mastodon gGmbH. This is exactly the scenario that Mastodon was supposed to be created to avoid, if you watch the 2018 official Mastodon promo it specifically mentions as its selling point that no one person or company controls it (https://fedi.video/w/cbQE3NRw76FayQCSdb14TU).
I realise recommending one server makes things easier for people who find servers confusing, but it seems a very unwise idea to recommend the supergiant server (for example mastodon.social is twenty-four times larger than mas.to, and mas.to is considered a large server). Instead, the front page of the website could recommend a rotating server from a pool of reliable servers with good track records going back many years.
A centralised Mastodon would be like a closed source Linux, totally pointless. Mastodon gGmbH needs to start spreading the growth like it used to if it is to keep its reason for existing.