I'm pretty sure there is already a thread on this...
Anyway, no. I don't see why, your starter pokemon, if you use it throughout the game, determines it's difficulty during the story.
Like in gen 1
Bulbasaur- Slower and easy to use and train, plus most gyms can be beaten easily,
Squirtle- A bit quicker but still fairly easy, has some weaknesses at gyms,
Charmander- Slower, takes time to raise and train, has probably the most weaknesses throughout the game,
I had to train Charizard up to almost Lv.80 to beat Lance for the first time, it was a very fulfilling playthrough.