I don't believe it's hopeless, but I do believe it can be difficult.
What I particularly do in this MU is first of all, choose a wide stage like, as mentioned before, FD.
D3 is usually content with wide stages so I don't think you'll have a problem choosing one as a CP or striking to a comfortable Neutral.
Throughout the match, D3 will of course be baiting for grabs and as well be expecting you to think the same. He knows you are going to be skiddish about the grab, it's a given. A buttload of easy damage sucks beeps, but I don't believe it should hinder our playstyle entirely. What I prefer to do is always have my "back turned" to the shorter end of the stage when possible. That way, if I am grabbed, not much profit will be given for our error. But I don't believe we should be as pansy-fied as it is led that we should. What I enjoy doing is playing an in-and-out game with D3.
One inside approach I use is a SH > airdodge inside of D3 where I end up behind him, immediately upon landing, I dish out a d-smash.
A second approach I like to use is when the D3 has a tendency of playing his air game and switching in between back airs and neutral B's. In this situation, I'm really trying to bait his neutral b by looking vulnerable to it, but then running underneath D3 and catch him with a pivot grab. Also, if you choose not to go under, but stay a decent distance away to where you know you will be sucked, but can react, then prepare a f-smash to punish or even spot dodge through him and deliver a d-smash.
Like told over and over again, if you get inside without much of an option, jabbing is a pretty hopeful way out. It doesn't always work, but it's definitely worth a shot if escaping doesn't seem like an option.
Like D3 and his tendency to throw out moves in the air in order to get us to approach (because he seems vulnerable) I really like throwing out safe, but seemingly punishable nairs that might just cause the D3 to approach, and as we all should know, if performed right, we can auto cancel the nair into w/e we please. I like jabs in that situation.
Also, if the shield of D3 is low just enough to see some of his fat body poking out, if you are bold enough to enter the lair where D3's gloves dwell, then I say try out a falling up air on him. And if it connects, you have a good chance of following up with an up-tilt, maybe 2.
Last thing, like someone else also said, D3 offstage > R.O.B. offstage. But it still doesn't mean we can't try things out in that realm. D3's like going for that bair stage spike a lot if you look vulnerable enough. So what I try to do is bait a whiffed bair from D3 so I can jump up and give him a bair of my own. Nothing wrong with experimenting.
It is a tough MU, but I feel the only way out is to try new things. Even if the numbers are against us.
I hope this helps in any way.