Well, the concept of this deck is quite interesting, but there's too many things trying to go on at the same time. When making a deck, you want to center the theme around only one or two things. Now, having only one of a card is pretty pointless under most circumstances, so we'll get rid of a type that's not really needed. The Grass and Water Pokémon seem okay, but there's too few of them, so we can get rid of those types, leaving us with Psychic and colorless. Now, You should add more Clefairy and Clefable, since those are great Pokémon, and maybe throw in a Cleffa for good measure. Drowzee is practically toilet paper material, so get rid of it. The three birds are okay, but you'd need more husky Pokémon to do good, and if we're going to play with Clefable, we need more trainers and powerful Pokémon to back it up. Since we still have the psychic energies available, let's throw in three Movie Mewtwo, since those are great. We don't need Kangaskhan if we have Cleffa, so let's put something else in as a good staller-- Chansey. She's colorless, and she's awesome as a basic.
Now for the trainers-- you HAVE to have Oaks and several Bills. Without them you just can't get through your deck quick enough. Throw in some extra Energy Removals (Super is best with Mewtwo Around), since those are good for putting off your opponent. Instead of the potions and Super Potions, add Gold Berries. You can keep the Lass, but you'd want some Comp Search and Item Finders to help you get Clefable out.
Here's what my version of the deck looks like:
PokéMon(13)
1 Cleffa
3 Clefairy (base 2)
3 Clefable
3 Movie Mewtwo
3 Chansey
Trainers(21)
3 Oaks
4 Bills
4 Gold Berry
3 Comp Search
3 Item Finder
2 Super Energy Removal
2 Energy Flow
Energy(26)
4 Double Colorless
2 Potion Energy
2 Full Heal Energy
18 Psychic Energy
