you can use whatever you want as long as you're having fun. fun is the most important thing.
of course, you will likely lose to anyone who uses better pokemon than you, but that is the nature of the game.
I can't tell if serious, but if you're suggesting a team running any Pokémon from a lower tier will probably lose to those running a pure tier team...
lol That's not how tiers actually work or predict who will win.
Certainly if you run a majority of your team from a lower team you are correct, but running some lower tier Pokémon on a specific tier team in no way influences such results unless it's a horrendously stupid choice. (Read Magikarp in OU) (Edit: Or totodile as TC suggested)
Only insecure and ignorant people try to debase someone online based on their username.