Online petitions for other projects, which usually have people in the hundreds of thousands, have little to no impact on the thing they’re demanding. I really doubt that a small percentage of people on the internet, who probably make up an even smaller percentage of the earth’s population, in that short amount of time, would have that much of an impact on a decision like that.
More than likely, Jeff Fowler, the director, who’s a really big fan, put in a really good word for Colleen to the people in charge of marketing and got her a credit. Or they just had to, since Tails is a big part in the film and the focus is more on the game characters this time around, and you can’t really market a character without crediting the actor/voice actor.