Mew is an extremely versatile Pokemon because it can learn every TM and HM in the game. has access to an enormous movepool. Furthermore, Mew is the only Defogger in the metagame tier with access to Will-O-Wisp and no Stealth Rock weakness, (I mean Latios/Latias and Scizor have no Stealth Rock weakness either.) which lets it cripple Defiant users such as Bisharp by halving their Attack from Will-O-Wisp. (Probably subjective change) Overall, Mew has good offensive and defensive stats but is not really that outstanding when compared to bulkier Psychic-types such as Mega Slowbro, regular Slowbro, and Cresselia. Mew has a pretty good Speed stat as well, which lets it outspeed outpace slower Pokemon. (period) and attack them. Mew also has access to a form of reliable recovery in Soft-Boiled, which allows it to recover health. contributes to Mew's longevity on the field. Synchronize is a useful ability for Mew, (RC) because it can can punish status users. (period) if they inflict status on Mew. (Idk but this seems redundant to me.) It can also provide utility against Scald burns. However, Mew's typing is subpar, which leaves it very vulnerable weak to common Bug-, Ghost-, and Dark-types, such as Scizor, Gengar, and Mega Gyarados. Mew also has suffers from four-moveslot syndrome, (AC) limiting its move choices. because it cannot benefit from all move options it learns. Despite Mew's incredibly huge movepool, it only has a few viable sets that aren't outclassed by other Pokemon in the tier.
Tried not to be too subjective on this. q.q