Probably for the same reason Simon dresses like he's going to a business dinner often when just hanging out on the ship of likely to be needed as a doctor.
Lesson from the times: Never trust an alligator to drain the swamp.

She likes to look pretty and is used to look pretty all the time.
A gentleman will not insult me, and no man not a gentleman can insult me.

You might also consider that as she has so much practice in making herself up it probably/maybe doesn't even take her very long to look that way.
I never imagined Inara had casual clothes. I know plenty of women who have such talent for hair and makeup they can look fabulous in 20 minutes. I'm not one of them, but they can.
It's also possible that it's simply part of the lifestyle/training of a companion to look and dress accordingly at all times. And Inara takes pride in her work, life, and self. She's very dedicated.
Undecided voters? been running for a year..like being undecided about Santa Claus & Dracula

Sometimes a girl just likes to look pretty for no reason.
How do you know it's love if you've never been in love before?

This was my thought also. Geisha would always have makeup, and intricate hairstyles that would be left in overnight, even using special neck supports while sleeping to keep their hair perfect. It seems like the companions live by similar standards.
Personally I loved seeing Inara's outfits, gorgeous costuming there, in a show that otherwise didn't have much opportunity for it since everyone else dressed very utilitarian, as is usual in this genre really. I dream that one day we will get a sci fi show set in some kind of opulent setting where everyone is dressed like Inara all the time

Look up 'Japanese geisha'and their lifestyle. A companion was partly based on the profession of a geisha.
Great white sharks are attracted to death metal music.

Right, because she is truly never off duty as she can be spotted at anytime, she is part of a guild of sorts, a representative if you will, like how she was presented to Preacher, but he misunderstood along with Mal mislabeled her as ambassador much for his own entertainment. She doesn't work 9-5 she has an image to uphold.
Wasn't me

That is correct.
A companion must maintain standards at all times.

That makes sense.
If it's all the same to you, I'll have that drink now.-Loki (Marvel's Avengers)

You're welcome!

That's the first solid suggestion - thank you!

I've always thought that it was a part of being a member of the Companion's Guild - you always had to look your best. I could be wrong, but it was actually a highly valued (and respectable) Guild with its own rituals and customs, so perhaps high standards were expected of its members at all times? Just a thought.

I'm sure it's just in her nature to look nice all the time, and perhaps she wanted to look good for Mal?
I am Senor Velasco, I drink my milk with tabasco

Yep. And dressing nice is a part of inarra's character. She's the respectab one in society.

Yeah, it is real criticism considering that costume is a huge part of character. Many actors say that well-designed costume helps them get into character. Stop being so bloody condescending.
As for dressing up for Mal - well, that's even worse. Not only is she apparently in love with a jerk who perpetually puts her work down and violates her privacy, but she wants to look sexy for him!

Complaining about a character always dressing nice? Yeah, that's some real criticism there.
Let's be bad guys.

I see your point, OP. Funny how when people love something they have a hard time accepting any criticism of it no matter how minor.

I just thought it was her way to dress. As simple as that.

Yes, Mal does bully her, and as everyone else on the ship shows, it's wrong. Mal is clearly in the wrong for thinking that's all she is. Especially since the show goes on to make a huge distinction about what she does.
That's not why he does it. Consider this exchange, from "Shindig":
Inara:
You have a strange sense of nobility, Captain. You'll lay a man out for implying I'm a whore, but you keep calling me one to my face.
Mal:
I might not show respect to your job, but he didn't respect
you
. That's the difference.
See, Mal doesn't trash-talk Inara about being a whore because he "thinks that's all she is". He does it because he knows there's more to her than that, and he believes she's better than her job. (And of course he's in love with her, so we can assume her sleeping with other men for a living really grinds his gears.)

You think he'd show up in some jeans and t-shirt? He'd still go in his sharp suits to work even if he had to change.
the show is dead set on portraying Inara as nothing but 'hot prostitute
Except that's not what a companion represents on the show. Several times Inara is shown to be the one that can counsel the others, even the preacher.
Yes, Mal does bully her, and as everyone else on the ship shows, it's wrong. Mal is clearly in the wrong for thinking that's all she is. Especially since the show goes on to make a huge distinction about what she does.
Let's be bad guys.

In the show he worked in a hospital as a surgeon, not as a GP. He would've worn hospital uniform, not his sharp suits.
My main point here is that the show is dead set on portraying Inara as nothing but 'hot prostitute' and the costume is a symptom of that. She is in sex object mode 24/7 and is constantly verbally bullied by Mal for her profession. Dressing her down occasionally would portray her as more of a multi-faceted human being that doesn't exist to please the onlookers.

You think doctors don't dress nice to go to work?

Because it's not Simon's work uniform as far as we know and he actually does start dressing down a little as he becomes more comfortable on the ship. Whereas Inara's outfits never let us forget her profession just like Mal.

Well, not everyone wants to look like a slob at home.
I don't see you complaining about Simon always dressing nicely.
Let's be bad guys.