I think the splat roller makes for a better side Smash attack. It would kinda work like Dedede's hammer. Slow, not as powerful, but also adds a little distance due to the paint flying.
Oh, sure, well maybe flicking it. But I think just rolling it and spreading ink on the stage is a great side-b. Would act sorta like Bowser Jr's, quickly cover space but have some endlag but good knockback. Since, well,
I imagine the Inkling (is there a better name for her?) would have a moveset that reflects her game! She wants to have as much stage control as possible by spreading ink everywhere. Then she can use...I guess down-b to transform into her squid form and travel through it, teleport through platforms maybe, but she probably shouldnt have her "choose where to travel to and fly through the air to get there" move from the game.
She could also hide in the ink like in her game! But maybe just for...2 seconds as a time. Should be used to dodge attacks, maybe travel a bit for positioning or getting behind an opponent charging a smash or something. But should have a lil bit of punishable endlag so you dont spam this. But also a little hitbox while rising, so you can still punish with a shield grab, but can't just wait for her to return and smash attack, yknow?
Also, your ink should only stay on stage for about... I guess 7 or 8 seconds, so you're game plan always reflects the game, ya wanna constantly demand stage control by covering it in paint. Or ink? Ahh, sorry, I think I wanna get Splatoon but I havent been following it super closely.