The practice of blackface and brownface is inextricably rooted in racial caricature and the use of racial caricature to discriminate and dehumanize. To engage in that legacy detracts from your cosplay— not to mention, it can also be hurtful to people who must contend with the effects of colorism in daily life.
"I think it’s one of those things where I pull my hair up, shave the sides, and I definitely need a tan. It’s one of those things where, hopefully, the audience will suspend disbelief a little bit." -Jackson Rathbone interviewed in 2009 on playing Sokka in the Airbender adaptation. (They didn’t end up tanning him or putting him in brown make up. Even the “The Last Airbender” production, for all it’s gender and race fail, didn’t think it was a good idea to go there. ATLA Sokka’s skin tone was not a tan.)
Don’t pull a Rathbone!