Your right in that it takes unnecessary effort to add a boss that's only going to be used once, though given they sink about 8 months to a year worth of time into each DLC charcater, they could if they wanted too. Really the Fighter Packs never promised anything other than the fighter, stage, and music, so I don't expect anything else.
As developers though they went an extra mile once already, making Spiral Mountain a fully modled stage in all directions, and choosing Banjo over the much easier to animate Steve XD. I don't think I'm overestimsting how much time it would take, several weeks or maybe over a month between however many animators and programmers they have left doing DLC. They already seem to be reusing Spiral Moutain for the boss fight, a stage that was done about 2 months ago. Smash bosses have extremly simplistic attack and movement routines so that part I know I'm not overestimating. Really just implementing Grunty's movement and attacks would be the long part, but again this game has several dozen assist trophies that do much of this.
Again I don't really even want them to spend the time to make Grunty a boss, I'm fine just her appearing in the stage and having Banjo to play as. But if other people wanna dream of a bossfight, I feel the odds are higher than normal for a DLC case. Especially compared to Joker and Hero where picking a original boss would have been more difficult, and somewhat pointless since the game had bosses that tangently fit them already. I can already imagine Grunty being an echo boss of Dracula and how quick that would be.