In terms of the cartoon itself, the main issue is the exaggerated features. The black-dark skin with the big grinning mouth creates this happy-go-lucky caricature of black people. Much like the portrayal of a happy slave in "Song of the South," this is clearly a racist hijacking of black identity.
Contextually it is horrific. White people made this cartoon to laugh at black people. Similar characters were acted out in black face. They were playing stereotypes to make fun of blacks. They didn't make the character mean or aggressive, so they could play it off as if it wasn't a harmful representation. But it is essentially a symbol of oppression. We have not only enslaved your people and dominated your livelihood and civil rights, we also get to choose how the world sees you. As a joke.
Wahoo isn't really much different. The United States did awful things to Native Americans and then decided it'd be fun to use their race as a cartoonish mascot for their teams, along with costumes and makeup and chants. Let's have fun at the expense of Indians because it's just fun and no one is being harmed, right? It's archaic and racist. As I've stated multiple times, I am not really holding anything against those attached to the logo or name. I get it (to an extent). You grew up with it. The name Cleveland Indians just sounds right, even to me. And hell, I personally LOVE the look of Wahoo as a logo. But I look passed that to the context of what it represents and it makes me embarrassed for the team and the city.
Wahoo wasn't created to harm Indians, but I don't think sambo was intended for harm either. That's really beside the point of what's wrong with them. The intent came from a place of ignorance and dealt harm anyway.
And this is without me reaching out for some article that probably goes much deeper on the issue and describes the problem more eloquently.