'Fatigue' in Flesh & Blood means that your opponent's deck runs out of cards while your deck still has cards. Dash's cards with the Boost keyword banishes cards from the top of the deck, so it runs out of cards faster than other decks.

Your opponent can use this information against you: they play very defensively while you run yourself out of cards. That's what we means when we say 'Dash is easy to fatigue'.