This does not apply just to Pokémon, but... Just because a boss is hard, it does not mean the entire game is hard, the same goes if you struggle against a specific boss.
Back in 2010, when I played Pokémon Platinum for the first time (I still have the save file), my knowledge of basic gameplay mechanics (e.g. type matchups, held items, status moves etc.) was very limited, thus I made some crazy mistakes, like equipping X items in my Pokémon or ignoring type matchups entirely in favor of just spamming a not effective move*.
*The worst example of this: Gardenia. For context, I chose Piplup as my starter, only trained him, and I didn't knew Flying-type moves were strong against Grass-types, so I got rid of Peck and my Starly was extremely underleveled (as were all the other Pokémon I had). The battle against Gardenia, and many other difficult battles I had, were won because of luck, rather than strategy.
Does this mean Pokémon Platinum is too hard? No, it's just me who didn't understood the game's mechanics, and I feel many people completely ignore this when discussing difficulty in Pokémon games, or even in games as a whole.