I don't think starters are all that manadatory, and there doesn't NEED to be a complete starter type circle in Smash. I would actually argue that Mimikyu is the more popular of the gen 7 Pokemon right now. It's actually in the anime at the moment, it has its own song, and there's a lot of merchandise for it at Japanese Pokemon Centers. If Nintendo doesn't want people to see under Mimikyu's cloak, that wouldn't be a problem at all (see the Japanese trophies for characters with skirts like Wonder Pink and Palutena).
I dunno, I just feel like Decidueye is a forced choice due to this idea that the three starter types need to be represented in Smash.
you know there is a difference between a trophy and a playable character right? with the trophy it's easy to hide some thing sure but playable? there is a whole new can of worms, it's not as simple as you say
anyway off that topic first Mimikyu having a song is not the special, other pokemon have one to as well
next sure it has merchandise but so does a bunch of other pokemon, heck the first merchandise of a gen is usually the starters
after that you bring up the anime, may i point out that greninja wasn't in the anime when he got added to smash, at that point ash only had froakie (who did eventually become greninja) ash also has a rowlet right now and while we don't know if they going to evolve or not, he still has it
and finally i don't think Decidueye should get in just because he is a grass starter, i think he should be in because he brings a new fighting style to smash, the only other series that could give us a archer character is FE but that ain't happening. Mimikyu doesn't really have that advantage, i don't really see how you could make it into a character beyond being a basic fighting game character, sure you could do some cool things with it's shadow has in it's animations but that doesn't make it a interesting character. Decidueye simply offers up more from a gameplay point of view than Mimikyu could