A+ -> A
Mega Mawile without a doubt is my favorite mega to use in the tier, and is one of my favorite mons in general. As an avid Mawile user, I don't believe it is worthy of its A+ ranking. I find it to be very close to Mega Scizor in terms of viability, but it makes more sense to drop Mawile rather than raise Scizor. First of all, Lando-T. The most dominant mon in the tier can switch in on Mawile at least once if healthy enough and threaten the OHKO with EQ and gain some free momentum if healthy enough to live a Sucker Punch. Speaking of which, Mega Mawile is extremely reliant on Sucker Punch to overcome its pitiful Sped stat. The Sucker Punch nerf has definitely affected this mon a lot. Mega Mawile is forced to run it, and Tapu Lele can flat out invalidate it. Thirdly, Mega Mawile's bulk is not great, despite having an excellent Steel/Fairy typing with the pre mega ability Intimidate, it is very underwhelming. This leads to Mega Mawile teams often built with Steel redundancy in order to actually utilize the benefits of a Steel typing, which can lead to stacking weaknesses. Mega Mawile also takes up the mega slot. Yeah, that's never been the strongest of arguments, and it's not meant to be a strong one, but it is another minor point against it. Recent metagame trends show an influx of Zygarde, which threatens out Mega Mawile and can live a +2 Sucker Punch after Rocks with 0/0 investment.
Of course, this is not to say that Mega Mawile is bad, it's far from it. Its insane Attack stat leaves it with very little switch ins, it is absolutely deadly under Trick Room, and if you can break Dugtrio's Sash, it 6-0's standard stall. Its typing is also fantastic, giving it two immunities, including an immunity to Toxic Spikes which is always appreciated. It has all the coverage it needs with Play Rough/Thunder Punch/Sucker Punch/Fire Fang/Iron Head, and it can even use Substitute and Focus Punch if it wants. Mons that supposedly resist Sucker Punch like Greninja flat out drop to it after very little chip. Sucker Punch also forces mindgames and 50/50's with the opponent, which is never a bad thing for you.
All in all, Mega Mawile is a strong choice for a member on your team, however, its flaws were understated by people (including me) when this mon was originally ranked, and it was ranked just a bit too high as a result.
C -> C+
C+ -> C
Everyone knows Starmie is bad by now and should really not be used. It has been brought a whole 0 times in SPL, and thus has gotten no results. Tentacruel, on the other hand, has been brought 3 times and won 2 of the 3. Tentacruel is much better than Starmie as a Rapid Spinner in the current meta, and the rankings should reflect this. Tentacruel is a nice check to Greninja, Keldeo, Celesteela, Mega Sableye, etc. while also providing hazard control in Rapid Spin/Being a Poison type and hazards in Toxic Spikes. It also, oddly, has a really good Speed tier for a defensive mon at base 100. Its lack of recovery and meh physical bulk definitely hurts it, but that's why I'm only nominating these two to switch spots. Hazard control is very limited in this meta. For Defog, you have Tapu Fini, Mega Scizor, Zapdos, Latias, and Latios. For Rapid Spin, you have Excadrill, Starmie, and Tentacruel. Zapdos is better without Defog, Excadrill is mediocre, Latios is mediocre. Latias and Starmie should not be used. Tentacruel is also mediocre, but its role compression, utility in Scald/Acid Spray/Haze/Knock Off, good special bulk and speed, makes it a much better choice for a Rapid Spinner than Starmie.
Tl;DR: Mega Mawile's ranking was overshot at the beginning by a subrank, and if you want a Water type Rapid Spinner, use Tentacruel and not Starmie.