Yes it is, because when people thing of $something, they assume the website is $something.com.
If Mastodon plans to appeal to the general public then it will need to be easier to find.
Most users of the internet have been exposed to TLDs other than ".com". For example, wikipedia is at a .org TLD, US government sites are at .gov domains, university websites are at .edu domains. Most non-US users will frequently interact with their country's (and neighboring countries') ccTLDs, like .de, .uk, .in, ... I find it surprising to assume that users of social networks who have already understood abstract concepts like "like vs retweet" or "like vs share" would find it difficult to understand the difference between .com and .social.
Also in a sense, it is more accurate for Mastodon to be at a .social TLD instead of a .com since Mastodon is a Patreon-supported FOSS project, and isn't a commercial entity like twitter.com or facebook.com. But yeah, I know that .com doesn't really mean "commercial" anymore, and is more of a general-purpose TLD now.
Mastodon has a number of issues that could stifle broader adoption, but I can't convince myself that the TLD is really relevant here. Most users will just be linked to Mastodon from other sites, or find it from a web search. Once its in their web history, web browsers will just autocomplete the site name in the address bar. And isn't the domain squatting and exorbitant pricing on ".com" the main reasons why the new TLDs have been released anyway?
I'd say most users won't type "mastodon.com" in their "browsers". They will type "mastodon" in their "internet" or, if technically savvy, into Google first. Second, from following the fediverse (not just mastodon, but also pixelfed, peertube etc), i have a feeling that they aren't into mainstream, general audience anyway. A lot of them are small focused instances and as such, won't even be attracting new people via Google, but by invites anyway. Many instances have closed registration anyway. So if you need to land anywhere it's likely not on mastodon.social, but something like ...(checks last five accounts to post directly on top of timeline): icosahedron.website, fostodon.org, mastodon.social, mastodon.technology (my instance) and hackers.town.
Ultimately this is kind of a problem, as we desperately need an general audience alternative to FB/Twitter that isn't about turning outrage into dollars. Right now I'm sure it's nice to hide from the Eternal September, but in the meantime Facebook is enabling ignorance, wasting everyone's lives on purpose, and proposing laws that only they can afford to comply with.
OP is about Keybase, which is trying to solve the problem of why the whole world isn't using GPG. I'm just pointing out that Mastodon has some public adoption issues still, despite the benefit that it can bring to the world.
You make a good point about how many users will simply Google the name, which naturally opens the question about whether users will want to scroll past info on the band & prehistoric animal before finding the service.
