Or maybe, from a perspective in the future, the steampunk era of spaceship design.

You must be unfamiliar with the Federation's backstory: A civilization on the other side of various singularities with both access to unlimited energy and the technology to transmute matter. As Commander Riker famously said, they don't even use money.

In the context of that kind of culture, I'd say the design of the Enterprise shows commendable restraint. No grand staircases and glittering chandeliers, the only gold leaf is on the uniforms, though I have to admit the staff is as well-groomed and earnestly polite as the crew of any luxury ocean liner in our era.

I recommend you broaden your perspective by reading some Iain M. Banks' "Culture" stories (wikipedia cheat sheet):

The Culture is a post-material-scarcity society formed by various humanoid races and artificial intelligences about 9,000 years before the novels in the series. Since the majority of its biological population can have virtually anything they want without the need to work, there is little need for laws or enforcement, and the Culture is semi-anarchist.[1][2] Its members live mainly in spaceships and other off-planet constructs, because its founders wished to avoid the centralized political and corporate power-structures that planet-based economies foster.[1] Most of the planning and administration is done by Minds, very advanced AIs.

The descriptions of their planetoid-sized starship habitats make the Enterprise seem decidedly shabby. They have grand staircases!