Mamoswine’s a great attacker, but its not the greatest (though some of the tier 1, aka mewtwo, also arent).
Mamoswine’s niche is agaisnt the double ice weak dragons or really just double ice weakness, against pokemon where ice is as effective against a target as other types, like Latios, Giratina, groudon, togetic, theres a number of other pokemon that are faster and better (aside from against groudon, where its third fastest v DT/EQ, but weak to its other 2 charges). Its ground moves are not bad, but against heatran its weak to iron head, fireblast and neutral to stone edge so very risky, while Heatrans also a PvP pokemon at the moment so not the greatest of targets, but raikou its a good budget counter for especially since Psystrike’s release has put people on edge about groudon investments but thats only going to be there for one week (and if its anything like the birds, who we havent seen for a year, similarly the beasts likely wont be back for a long time after this). As a gym clearer, its good against the double ice weak dragons, but isnt an all star gym crusher like metagross or machamp for more than just those.
Zygarde, also may not be the greatest of pokemon, since it doesnt have the same top tier stats as the other dragons, and only the addition of its signature moves could really save it from being something beyond a target to beat raid rewards out of.
Mamoswine’s got a lot of good qualities when it comes to countering pokemon like rayquaza, and is at the very least a good option in a larger number of match ups, but beyond the double ice weakness (of which only 3, 4 if including mythicals, legendaries have), it does lag behind other pokemon. Its typing’s also pretty poor, so much to the point its use as a ground type is nigh nonexistent as its neutral to rock unlike other grounds, and is weak to steel and fire (though its not like ground is best for these types anyways) but can be a good option against electrics, despite their rarity as higher tier raids, and as an ice attacker theres strong competition for flying coutners (as well as only 3 flying types ice is best for, while rocks beats it against almost everything else flying) its weak to grass and theres few targets of this, ground’s also relatively scarce, but holds supreme effectiveness agaisnt 2 of them for legendaries, while nonlegendaries tend to be more weak to grass/water, and as for dragons, it only stands out agaisnt the double ice weak ones, or prior to dialga’srelease as a strong coutner that wasnt weak to dragon (though dialga’s release, and the dodge bug being fix has largely nullified this outside snowy weather).
In short, It’s strong yes, but doesnt pack the neutral DPS and shear bulk metagross has, or the reliability in being best for gym battles and nonT5 raids machamp does, and many matches its super effective for, others handle better, aside from double ice weaknesses.