One of the things I've always liked about the core concept of the United Federation of Planets
was that it is an interstellar community of diverse and distinct species (humanoid & non-humanoid) that find ways to work cooperatively and collectively despite the multitude of differences which exist between them.
Common cause and all that.
One of the best examples I can point to in the books of a non-humanoid character just "being there" and it not being 'special' is P8-Blue
" the Nasat structural engineering specialist aboard the USS DaVinci. Always loved how she was just one of the girls as it was.
I'd have to agree that while it probably wasn't intended, the Titan novels (in part) have had a bit of an "after school special" sort of vibe in regards to the diversity of it's crew. Maybe the issue, is that because the stories are meant to be encapsulated and accessible between other books, the diverse nature of the crew is something that has to be pointed out again and again. From my perspective though (as both a ST fan and a long time reader of the Titan novels) it's unnecessary as the fact that the crew is diverse shouldn't be a big deal from a Federation perspective.
Starfleet is a diverse multi-racial peacekeeping and exploratory organization by charter and so if a character is for example a sentient plant, I'm okay with that.
I don't have to be told how special having a "sentient plant" is over and over again. I'd be more impressed if it was hardly mentioned at all as that would be more indicative and in keeping with the 'Federation attitudes' that exist in my head.
Like others here have stated, I'll agree that within Titan
itself, Christopher's books have been the best about establishing the Titan's crew as characters first (who just happen to be members of species whatever,
while simultaneously not taking anything away from them because they're species whatever
[Read as: They're neither white-washed or stereotyped]
On TV, we mostly saw humans, and humanoid aliens, but I just attributed that to budgets.
Hopefully future novels can foster a greater 'sense' of Starfleet crew diversity without it being pointed to as an example of diversity, but rather the status quo. (if that makes any sense whatsoever).