There are some things that make it feel a bit dated/cheesy (the dolphin officer, mostly)....
I'll never understand why people think that. There's overwhelming evidence that dolphins are sapient beings whose mental capacity may match or even exceed our own in some respects. You'd think SF fans would be thrilled by the idea of real alien intelligences coexisting with us here on the same planet. It's the sort of thing that should inspire awe and wonder in the lover of SF, and it's an idea that's been explored by many respected SF writers over the decades, from Arthur C. Clarke to Larry Niven to Anne McCaffrey to David Brin to Douglas Adams to William Gibson -- and by nonfiction science writers such as Carl Sagan. So I'm always bewildered when I come across the reaction that it's somehow silly or cheesy to portray intelligent dolphins in fiction.
Stuff like seaQuest DSV and the 80s whale craze probably tainted the idea for a lot of people, although I would imagine seaQuest and its horribly "cutesy" voice for Darwin had the more negative impact. And I think the gradual jading of our society probably soured lots of people on the idea, especially when dolphins would wind up stranding themselves and dying - it's hard to keep your hopes that dolphins were really intelligent alive when they keep beaching themselves for no apparent reason.
With Dark Mirror, I think the problem is more about the image of a dolphin flying around in a water-filled space suit thing than anything else. It just seems like a really ridiculous image and concept to swallow, one that just ruins suspension of disbelief because of how absurd it is.