1 VS. 1 is where Charizard is good.
2 VS. 2 is where Charizard is bad.
I do not get where the idea of the opposite being true comes from.
The only way Charizard can be decent in 2 VS. 2 in my opinion is if Team Attack is off, as in that case he can not only freely use Flamethrower without fear of catching his teammate in it, but his teammate can also abuse the hitstun caused by Flamethrower to attack the opponent.
In Smash, characters can be put in one of the two following categories: Chaos-based characters, and order-based characters.
Chaos-based characters become more viable the more "chaotic" the environment they find themselves in. These characters do better in FFA and Team Battle than 1 VS. 1, and most likely even increase in viability the more players you add to the match (like in 3 VS. 3 and 4 VS. 4 via 8-Player Smash). Chaos-based characters generally succeed in dealing most of their damage independent on stage control, edgeguarding, spacing and combos (not to say that they cannot do these things well, but they can be successful even without them), and also generally hit very hard, with some moves that are extremely rewarding but are very hard to hit a good player with in 1 VS. 1, and also rarely if ever leaves themselves open to being attacked by one character while they are busy dealing with another. These characters greatly appreciate the chaos of having more than two players on the stage and the opponents having to focus on more than one character, which makes it easier for them to be hit by powerful moves that may not be practical in 1 VS. 1 where the opponent has only one character to focus on. Two examples of chaos-based characters are Captain Falcon and Ganondorf. They hit very hard, have devastating moves that are much easier to land with in a chaotic environment, and a lot of the damage that they deal comes in a single, quick hit, meaning they do not have to leave themselves open to being attacked by a third character while performing a combo or anything like that, and both of these characters also don't rely on stage control for success, especially in the case of Captain Falcon, who has sheer mobility as one of his greatest strengths.
Order-based characters become less viable the more "chaotic" the environment they find themselves in. They perform optimally in 1 VS. 1, but fare more and more poorly the more characters are added to the match. In 1 VS. 1, these characters succeed mostly by relying on combos, edgeguarding, or means of stage control, but all three of the aforementioned means of success become far less applicable outside of 1 VS. 1 due to the possibility of being attacked by a third character while edgeguarding or combo'ing another character, and stage control as a concept pretty much goes out the window or is at least heavily altered when there are more than two characters on the stage. Order-based characters also tend to rely on spacing a lot, something which is very hard and easily disrupted in a chaotic environment. Two examples of order-based characters are Charizard and Rosalina & Luma. A large amount of the damage that Charizard deals in 1 VS. 1 comes from Flamethrower (through edgeguarding or otherwise), spamming jab and using pummels/throws, all of which leave him open to being hit by a third character, and with Flamethrower and Nair spacing and all as well as his edgeguarding, he is also obviously a very stage control-based character. As is Rosalina & Luma, one of whom's major strategy involves positioning Luma properly on the stage in order to make safely landing or approaching difficult for the opponent, and a more chaotic environment is especially debilitating for this character as Luma tends to die more quickly in such matches.
is one of my beloved characters. I'm a huge Pokémon fan and he represents the best Gen imo -- Gen 1!!!! (Though 
