The abomination of knuckles in boom aside, his general thing has stayed about the same, with a few tweaks here and there. More so than tails. Nothing is consistent in the sonic universe.
There's actually more then just Boom. There's how he was in the Adventure games, perhaps his best portrayal where he is the guardian of the Master Emerald and while he still makes his mistakes (Especially in SA1) he's still generally competent and has a solid goal.
...Then that's completely thrown out in favor of Knuckles joining in everything without ever mentioning the Emerald started with Sonic Heroes.
Probably doesn't help he gets a few idiot moments like running towards a dead end and then turning the right way in one of the last cutscenes in Sonic 06.
Maybe some character moments haven't been the best for Tails either like in Lost World. But he's still done more substantial things then Knuckles has in recent years. (Heck, Going back to Lost World. Knuckles just stands around as he and Amy are frozen(?) from that life-sucking thing)
Though honestly character portrayal should have nothing to do with in regards to Smash. Tails is Sonic's Luigi, he offers the least complicated as well as having more unique properties to him. (Knuckles unique properties may be gliding and/or even climbing, but otherwise I think he'd just be made a heavier, stronger, slower Sonic. Like how Ganondorf is to Captain Falcon. Tails would also share some moves with Sonic but he'd also have a far better recovery and the possibility of using his plethora of gadgets. He'd probably also be lighter (Though not as fast as Sonic, but still generally fast like maybe around Captain Falcon's Speed)). As for Eggman, while it'd be cool to have him the only way I feel he'll work is if they base him off Bowser Jr. with Eggman in his Egg Mobile. But I feel having a character that can work similarly to Sonic while still generally nearly as popular and iconic as Sonic as the 2nd Sonic rep would be easier on the devs. Cause even if they go for the Eggman Jr. route, they'd still probably have to work out unique things for his moveset cause it's not like everything in Junior's moveset is something Eggman ever had.