I think it COMPLETELY depends on the person AND the use.
"Lock o'they Eyeraische" - irish
"Bloody Hell." - brit
"Wazzup wazzup." philly
'Meat-a-balls." italian
"Whasah goh'shoo so down low, cher?" cajun
"Wail Pahhhhrdnerrr." bad John Wayne (aka how most people imitate english
"Zis eez ze deesh of ze day, and certain e moi, you vill loave eet." french
"Bruthas and ah sistahs! Lemmme HEAR ya RAISE your voices!" southern black baptist
"Honto?!?!" japanese
"Ooooooh my back! You know in MY day kids showed some proper RESPECT to their elders and didn't LET them win games as if they were all senileified and incompetant!" my great aunt to a T.
"Eeef I were a reeech mahn, deedle deedle deedle deeeee." fiddler on the roof
These are all phrases that have been heard in my house this past WEEK, from . (I'm trying to survive raising Robin Williams, apparently. My son is dead on mimic.)
Mimicry can be designed to hurt, comfort, entertain, bring J., cause pain, educate or learn... it is not in and of itself racist. It can certainly be used that way, but it's not cut and dried.
Of course, I have an uncommon background... since I spent most of my life as a minority (white chick in asia). But I typically judge via the PERSON, and not, say, just the mimicry.
CERTAIN things are dead on racisit, sexist, homophobic, etc... but I wouldn't categorize mimicry as one of them, even though it can be used that way.