Excuse me? Even if you ONLY used Charmander, it is at a disadvantage in 43 (roughly) of ALL the Gym/Elite Four Pokemon match ups with only around 17 advantages. (see the Nerdist's Because Science video on the subject.)
That is only if you naively compare types.
Brock's Pokemon don't have any Rock or Ground moves and they have very poor Special, so Charmander actually has the advantage here. He actually does better than Bulbasaur here.
Against Misty Charmander does have a straight up disadvantage. No getting around it.
Against Lt Surge Charmander can learn the Dig TM and get the advantage.
Against Erika Charmander already has the advantage.
Against Koga Charmander again gets the advantage with Dig or Earthquake if you taught Dig to something else.
Against Sabrina Charmander is neutral, but still does better than the other two starters thanks to Slash.
Against Blaine Charmander has the advantage thanks to his Fire resistance and Dig/Earthquake.
Against Giovanni, Charizard is immune to Ground attacks and particularly Fissure and I don't think any of his Pokemon have Rock attacks. Furthermore, Charizard again has Dig/Earthquake.
Against Lorelei, I think Dewgong doesn't have any Water attacks, so you could use X Accuracy and Fissure to easily win this fight.
Against Bruno, you have type advantage against his Fighting Pokemon and the Onix are so terrible that no team should have trouble with them.
Against Agatha Charizard beats four of her five Pokemon with Dig/Earthquake.
Against Lance, Charizard again has the disadvantage, particularly against Aerodactyl.
If you're playing smart, Charmander actually has the advantage against all Gym Leaders and E4 members except Misty and Lance. Compare this to Bulbasaur, who sucks against Brock (thanks to him getting his first Special move at a relatively late level), Koga, Sabrina, Blaine and Agatha. Even Erika is a bad fight if she decides to spam Wrap. The only fights Venusaur has a clear advantage against are Misty, Giovanni and Lance (thanks to AI quirks causing him to spam Agility against Poison types).