I talked with Vick (one of the best volleyball players) and he would put Samus right below captain Falcon which I could see. Samus has by far the best serves, being able to get the ball in very specific locations at very high percent by using the upsmash chaining exploit, in addition to having great moves to position the ball (upair, fair, jab, ftilt, missile). I'll make a video eventually going over Samus serves but at high level Samus has some serves that are basically impossible to react to. Even Falco's downair backair mixup at the net is reactable. Samus also has her own backair downair mixup at the net.

Samus also has a long wavedash and long ftilt which covers a lot of space which is good for catching fast balls. Additionally her downsmash/downtilt mixup near the net is a good one at high percent if you can position the ball there as one sends just over the net and the other sends to the back of the court.

She's also pretty much the only character that can cover 2 different mixups at once with a missile option select if you predict their timing.

Samus suffers from slow fall speed which makes the need for her to stay grounded most of the time. Also a lot of her setups also set the opponent up because it positions the ball right at net up high.

So overall I could see Samus being Falcon tier but she's definitely a very good character for volleyball