To be honest, both of them are not exceptional in competitive game play, and there's almost never a reason to use them unless you happen to like them, because they're outclassed by other Pokemon in either of the two niches they fill: Offense or Support.
For higher tiers, Meowstic - Male is slightly better than Meowstic - Female, whereas the reverse is true in the lower tiers.
Male Meowstic is slightly more useful because of its ability Prankster, which lets it set up Light Screen/ Reflect at +1 priority, so it can typically guarantee at least one screen before fainting, but it can set up both, and further cripple the opponent with either Yawn/Thunder Wave. That's about the most use it has, specially if you're up against stronger opposing Pokemon in the higher tiers.
A typical move set would be:
Meowstic (M) @ Light Clay
Ability: Prankster
EVs: 252 Speed/ 252 HP/ 4 Defense
- Light Screen
- Reflect
- Yawn/ Thunder Wave
- Psychic / Fake Out
Another alternative would be:
Meowstic (M) @ Choice Scarf
Ability: Prankster
EVs: 252 Speed/ 252 HP/ 4 Defense
- Light Screen
- Reflect
- Thunder Wave
- Trick
In this case, your strategy is to catch opposing set-up lead (such as Shuckle) off guard and Trick a Choice item to it so that it is forced to switch, at which point you can set up your own Screens.
Female Meowstic is more offense oriented, but even so, its stats are quite sub-par to be a major threat except in the lower tiers with other weaker Pokemon. A Life Orb/ Choice Specs set with 3 attacks + Trick (typically Psychic or Psyshock or Expanding Force/Thunderbolt/Dark Pulse/ Trick) is where it is most effective, working as a potential late-game sweeper or to quickly cause as much damage to an unprepared opponent as possible before fainting.
Keep in mind that you can check their movepools to see how else you can use them, based on the rest of your team. If you so prefer, you can have them set up Psychic Terrain or Misty Terrain to further support your team as you see fit.
One final bit of caution is that if you do want to play in Nat-Dex AG, there are far better Pokemon than them, so use them only if you like Meowstic.
