I can't see a reason to use RD Seismitoad alongside Mega Swampert, Seismitoad doesn't even beat any of the pokemon that Swampert struggles with (both offensive and defensive Celebi, Tangrowth, Alomomola, Suicune); while pokemon like Ludicolo and Kabutops have the STAB/boosting move/power to heavily dent most or all of these pokemon, while also doing better versus general offense than Seismitoad thanks to more power. Seismitoad doesn't even really work as a one-two punch vs Grass-types because they heal to full with Giga Drain; Ludicolo seems like it straight outclasses.
That said, I don't believe Seismitoad should be unranked, though maybe move down a rank; this isn't my set, but I've used it quite a bit:
Basically trades Swampert's improved bulk and Roar for the ability to switch into Water moves, including Scald, which is actually huge in practice - for example, it doesn't care about Tentacruel because it can't be Scald burned and EQ 2HKOs, whereas burned Swampert's EQ is a 4HKO, letting them do all of wear you down + spin + get T-Spikes if they want to. Water Absorb is enough to force out Suicune early on so you can get SR, whereas Power-Up Punch guarantees to beat lastmon Suicune, the advantage of which over Swampert's Roar I can't stress enough, since it's not always easy to fit Celebi/Toxicroak on every team (maybe the former haha, unless you have a grass/psychic already). Sadly it gets overwhelmed a lot earlier by Entei/Mega-Aero, so it's basically role compression for more offensive builds who aren't expecting 150+ turn games. Still, I think it deserves at least C (this point is often made but we have stuff like Honchkrow and Yanmega in C+, either they need to drop or other mons need more credit)
That said, I don't believe Seismitoad should be unranked, though maybe move down a rank; this isn't my set, but I've used it quite a bit:
Life is suffering (Seismitoad) @ Leftovers
Ability: Water Absorb
EVs: 248 HP / 252 Def / 8 SpD
Relaxed Nature
- Stealth Rock
- Scald
- Earthquake
- Power-Up Punch
Ability: Water Absorb
EVs: 248 HP / 252 Def / 8 SpD
Relaxed Nature
- Stealth Rock
- Scald
- Earthquake
- Power-Up Punch