I don't really think Liepard is very good to be honest for quite a few reasons
- Although Meowstic-M gives some competition, it is notable that Whimsicott generally completely eclipses Liepard as a Prankster user. It has many of the same moves (Knock Off, Foul Play, U-turn, Stun Spore which is pretty similar to Thunder Wave, Taunt, and Encore are the more important ones), a MUCH better defensive typing in Grass/Fairy, OK bulk that is much better than Liepard's, and some form of recovery in Leech Seed and possibly Giga Drain.
- Very poor defensive typing coupled with a similarly poor bulk. Not sure what the ideal Liepard spread is, but everything in S or A OHKOs 252/0 Liepard aside from Froslass (supporter), Alomomola (its a freaking Alomomola), Cresselia (2HKOs with Moonblast), Banette, and some Zoroark (if they have LO Low Kick or Focus Blast, it loses). This wouldn't be a problem if it was an offensive Pokemon but support Pokemon shouldn't have this much of a problem switching into the battle. The less said about 0/0's bulk, the better.
- Liepard unfortunately is barely slower than Virizion, Cobalion, and Durant in the scenario you would want to go use max speed (which is almost all the time probably). Cobalion and Virizion even have Justified to make sure that Liepard's Knock Off or Foul Play gives them a boost to their attack.
- Its not the most fun thing to switch into, but if its using Foul Play (you probably shouldn't be using Foul Play over Knock Off, its attack isn't bad) special attackers have a pretty easy time manhandling it. Knock Off is annoying but... everything with Knock Off is pretty annoying. In regards to just using Knock Off Liepard is statistically inferior to Zoroark in almost every regard but Zoroark has a much better offensive movepool. It's probably worth mentioning Zoroark is bulkier than Liepard. That's just sad. On that note there are a lot worse Pokemon to switch into, and with Liepard's movepool being limited to Dark moves and U-turn, there are much worse things out there.