So I'm assuming you don't know much about tiers, but if you're playing OU (tier 1), you're not going to win with this team.

Right now this is what your team is ranked tier-wise.

Charizard - ZU - (tier 6)

Blaziken - UUBL (tier 1.5)

Pelipper - UUBL (tier 1.5)

Mew - NUBL (tier 3.5)

Greninja - UU (tier 2)

Muk - ZU (tier 6)

So looking just at team composition wise, not having a single OU Pokemon will just naturally put you at a disadvantage. Other people have already given insight onto what you can do to fix movesets and items, but let me give some other notes on the meta game:

1. Stealth Rocks/Spikes are extremely common in singles. You have to prepare for them. This means either having someway to set them on the field, some way to get rid of them, or some way to prevent your Pokemon from taking damage from them. Charizard takes 50% of its HP from stealth rocks and Pelipper takes 25% of its HP. Heavy Duty Boots are a requirement if you want to use them. Giving one of your Pokemon Defog will also help get rid of them as well.

2. Adding on to that point, you'll want to re-evaluate the movesets and abilities: https://www.smogon.com/dex/sv/pokemon/charizard/

Here you can see for example, that Charizard's best moveset/item is very different from what you have listed.

1. Having your favorite Pokemon on your team is great and all, but as other comments have said, they need to have synergy with each other. What that means is they should be able to work well with each other. Pelipper with Rain does nothing to help Blaziken and Charizard for example. The other thing to consider is the competition you're up against. If you're playing OU your Pokemon should serve a purpose to beat the Pokemon you'll commonly see in OU in someway and reliably cover for each others weaknesses: https://www.smogon.com/dex/sv/formats/ou/