Derpy to me is not offensive in anyway possible, she is actually a very good symbol of the show. She is a pony that really makes our community stand out if anything. Derpy is something that brings our community together, when she made her first real appearance in the show we all celebrated together. But when her voice and eyes were changed, we all cried together as well. To be honest I don't see how anyone could think of her in a bad way any way possible. She is a cute background pony with adorable eyes, that's all.
Her voice though some people find offensive though. To me I don't mind the voice at all, but that is just me. There are a lot of people that I have met that did find her voice very upsetting to them. I find the voice though very sweet and silly. It's a cute little add to her character to my opinion.
Of course, everyone has their own opinion on Derpy with her eyes and voice. This is just mine.