Amiibos are pretty much DLC in the shape of physical media. So yeah, the amiibos (not only the Splatoon ones) are pay walls for content. Sadly people love dem toys and will buy it regardless of how the success of amiibos affect the design of Nintendo games. I mean, so far one can argue none of the games released after amiibos have any meaningful content locked behind the toys*, but who is to say Nintendo is going to lock even more content behind amiibo in future games? I mean, why shouldn't them? That said, I did buy the Squid Girl amiibo so I can't really judge others for supporting amiibos anymore... I just wish the locked content were made available as regular (digital) DLC as well, perhaps a few months after the amiibo release, so people who live in places not named USA/Canada/Europe/Japan can have better chances of accessing the content.
*Which I disagree, since a whole game mode in Mario Party and multiplayer usable outfits + mini games in Splatoon seem to be pretty meaningful content to me.