For splatoon it is a bit of both.
Amiibos lock too much in splatoon that should be standered (playing different weapons in 1 p)
now what they should of did is maybe have the different challenges (like kraken time trail mode, ink save mode) for all the stages. Doing that with allowing the use of different weapons would greatly increase the value of 1 p.
the amiibo gear prob should been with base game too while the amiibos could of had different color changes for them and weapons.
Something like mk8 does it right, the amiibos where not spifcially for that game while it gives a nice bonus for buying them for smash or something.
This is why splatoon amiibos needed to work different since they are pretty much for that game. They do not transfer well to others.
They do what they're intended to do: make people buy DLC without realizing that they're buying DLC. In that regard, they are truly brilliant. Nintendo looked at the negative attitudes people developed towards paid DLC and started selling what looks like a collectible with bonus content. It's amazing the difference it makes when you take something people aren't thrilled about, repackage it, and market it differently.
People do not like DLC because a lot of times, you are now paying for an unfinished game for the same price as a full game while paying MORE to actually have the finished game, kinda like why splatoon amibos are attacked.
DLC done right, look at MK8, no one complains about MK8 dlc.