This is true, but these Formes will likely increase the pokes strength in some way shape or form.
There are other forms that increase a Pokemon's strength is some way, shape or form.
Giratina has a Stat shift, redistributing his stats and making him stronger in some areas.
Shaymin has a Stat shift, redistributing his stats and making him stronger in some areas.
Deoxys has 3 stat shifts, redistributing his stats and making him stronger in some areas depending on his form.
Rotom gain additional stats.
Kyurem gains additional stats.
Arceus (while not officially a form change) increases the power of his attacks per plate.
Castform has a boost with his form changes adding STAB to some attacks and boosting his signature attack.
Wormadam all have different stats depending on form when they evolve into their current form.
Cherrim gains a boost when it changes form.
The Genie trio all have stat redistribution making them stronger in newer areas than before.
Other than Rotom and Kyurem, they all don't change in original strength as their base stats don't change, but thanks to the stat changes involved or indirect boosts due to required items and abilities they are made stronger while in their new forms.
Closest relation to the Mega Stone is Giratina, a hold Item that is required for the form change and thanks to the hold Item the Pokemon specific to it gains a boost. Its no different than any form change before in terms of how it effects the dynamics of a Pokemon. Only differences is in what we call it and the amount of different Pokemon that share the same ability.
Unless a form change is completely aesthetic like Unown, Keldeo, Shellos, Basculin, Pichu, Spinda, Deerling, and Sawsbuck, they have some kind of effect on the Pokemon's battle performance. (though most of these are variants and not special forms. Arceus and Genesect are both considered Variant forms though since their items boost their signature attacks and since Arceus gets more bonuses from his plates I'll include them as traditional form changes for this.)
I left off a few...didn't really see a point in naming every single pokemon for this.
Muks half fairy! its been confirmed :o
Please don't just make things up on a whim. No Muk has not been confirmed to be anything but Pure Poison. The only confirmed Fairy Pokemon are:
Sylveon
Xerneas
Spritzee
Swirlix
Gardevoir
Marill
Mawile
Flabebe
Dedenne
Jigglypuff
Nothing else has been said to be Fairy Type.