On the subject of shill picks, I think the key to their reception is one that applies to all kinds of marketing: awareness. If people are aware they're being marketed to, their opinion will lean towards the negative from the start. So the trick is to advertise to people, while simultaneously hiding the fact that you are.
In the context of Smash, that basically means picking the right stuff to shill. If your character has other reasons to be in besides advertising, then people will probably latch on to those as the justification rather than the promotional aspect (or at least, the people who like the character will). Conversely, if nobody can think of any other reason for the character to be in, then they'll be seen as pure promotion, and thus a "shill pick". In that sense I think we can consider "shill" the polar opposite of a "deserving" character in the context of speculation.
One example of successful promotion was Isabelle. Her trailer was literally framed as an announcement for a new Animal Crossing, and immediately followed by the actual announcement. It was impossible not to associate the two things or to speak about one without the other, so it put a huge spotlight on the game. Regardless, few will argue that if New Horizons wasn't in development, Isabelle wouldn't, or shouldn't, have been in, so the promotion was seen more as just killing two birds with one stone. Plus, Animal Crossing was seen as both a franchise that could use the promotion, and the extra characters. That, to me, is the example to follow (of course, there's other examples of promotion done right, but Isabelle might be the best modern one).
