Mime is not just anti-Kingdra, first of all. I've already talked about why this card is good against anything relying on a 1-2 energy (or 0 such as Bronzong/Yanmega/Rhyperior lv. X etc.) attack in previous posts and I don't feel like repeating it. Just realize it is not included here solely as anti-Kingdra.
Furthermore, as anti-Kingra, it is pretty exceptional, and I think this has also already been explained pretty well by Papi/Manny, rroaryh, and others. I'd just like to reiterate that if nothing else it SLOWS KINGDRA DOWN while also giving them less energy in hand to ditch to get the most out of Dragon Pump, which is a blow considering speed is one of Kingdra's strongest points. One Mime will force them to power up at least 1 Kingdra with 3 energy. Who says that Kingdra doesn't get KOed before the Mime does though? If Gliscor lv. X got a Night Slash or Pester in previously, it's wide open and vulnerable. If the Lv. X hasn't even come into play yet, it can lock down the Kingdra with Paralyzation and hit it with Night Slash only to come back the following turn for the KO. If a Kingdra goes down while Mime is still in play (or if 2 are/another is brought out), the next Kingdra is going to have to go through the same 3-turn procedure while Gliscor lv. X is waiting to continue its hit-and-run games, setting that Kingdra up for a KO as well as it struggles to get 3 energy on it. With the Mime and Gliscor lv. X combo, you essentially rob Kingdra of its speed and pass it on to the Scorpio player, forcing them to wait to even attack while you still get 2HKOs in.
The whole point of this deck though is really to abuse the hit-and-run nature of Gliscor lv. X comboed with Venomoth's status-inflicting Body (which is actually why I favor 3-3 Moth and only 2 Mime, although like Umbreon777 himself I have yet to actually test this thing). By switching out to Venomoth after a Night Slash, odds are the defending Pokemon is going to get poisoned unless they're a sniper (and these have been acknowledged as a threat since the first page of this thread) or unless the opponent uses Warp Point, which they are probably going to regret doing later on when they've been Paralyzed by a recycled Gliscor lv. X and have already used up their 1-3 supply of switch cards. 60 + 10 from poison + 60 from Night Slash + 10 more from poison= 140, which is enough to KO 99.9% of the Pokemon in the DP-on format, obviously including Kingdra. If the Pokemon attacking Venomoth is fresh and damage-free, Pester can be used to OHKO it next turn if it has 100 HP or less, and this is actually pretty useful considering all of the playable/hyped Pokemon out there that fit this bill (Bronzong, Yanmega, the Pixies, Froslass, Groudon, Kyogre, Lanturn, Tentacruel, Banette, Lucario, Bellosom, Glaceon lv. X w/out Espeon back-up, etc.). Luxray and Blissey also both go down in one hit from a Pester set up in this manner due to weakness. With PlusPower/Buck's, you stray over into the 110 HP territory, which hits stuff like Magmortar, Infernape, Leafeon lv. X w/ out Espeon back-up, etc.. Pretty handy if you ask me.
The best thing? Gliscor isn't taking damage all the while that he is inflicting it.