You're talking about the smash fanbase.
The most entilted fanbase in the whole universe for the single fact that this fanbase is equal to multiple fanbases of different franchises togethers.
Trust me, if you though that Corrin was bad and a "shill pick" trust me, this fanbase will tear a new one for the new pokémon rep if it's the next DLC character ESPECIALLY because it's one of the biggest franchise in the world.
I doubt a new Pokemon will get the level of hate to the extent of Corrin/Byleth/Fire Emblem.
Will it get backlash?, Sure, But every character is prone to getting some form of it whether it's big or small because you can't please everyone.
Will it get praised to the extent of Banjo-Kazooie or Joker? I doubt it, It will likely come down to what Pokemon they choose.
At most it would be neutral because a lot of people are kind of expecting it to happen and even if there are fans that are getting sick of too much Pokemon, There's no denying that along with Mario, Both are Smash's biggest franchises that have the longest reach and massive audience appeal that helps offset some of that hate. The same can't be said for Fire Emblem.
Many hate seeing Fire Emblem getting special treatment similarly to Mario and Pokemon despite not being in the same league, It also doesn't help that the characters play and look very similarly to each other which is a big reason why they get the most criticism ,Where as Pokemon have way more different archetypes, looks, aesthetics and have a wide pool of attacks and movesets to work with that really set them apart.
Bottom Line: A Gen 8 Pokemon will probably get decent/solid amount of criticism and result of a middle of the pack pick that isn't viewed as exciting, But I doubt it will reach Byleth/Fire Emblem levels.