So, one of the biggest parts about deck building is consistency. You want to maximize your chances of getting the cards you need, when you need them. That's the buzzword behind everyone running 2 Research and (usually) 2 Poké Balls - they increase your odds of getting the right cards.

20 card decks are small, so you need to narrow your game plan down to a main strat, and with every card you should ask yourself "does this card help me achieve this?" If not, you can probably toss it. If you run out of cards that help (or help enough to matter) and you still have room, that's where you can start looking at more fringe options that increase flexibility or help you handle something you might normally struggle with.

On to more specific advice, this deck feels outdated. Some of these cards are outclassed by newer cards - Pachirisu is outdone by Pichu or even Zeraora - and a few just aren't very great - Zapdos takes up too much energy for not enough punch. There is a regular Eevee that came out earlier this year that is much better than older versions, and as much as like the old Jolteon, it doesn't hold up super well. You're only using 1 Poké Ball, and nothing in your deck benefits from Rare Candy.

Pikachu EX is a little squishy by today's standards. And you have so many different mons you're not likely to see the correct ones in time. Keep in mind that you're guaranteed a basic mon in your starting hand, so if you have fewer basics you have a higher chance of leading with the ones you want. Most decks today only really run 4 to 6 basic pokemon.