Is there an aphorism or proverb in English which describes attempting to improve something fundamentally flawed by dressing it with a lot of ornament?
If it's fundamentally flawed: you can't polish a turd.
There's the relatively recent one about putting lipstick on a pig.
I'd never heard of lipstick on a pig before US politicians decided to popularise it very recently, so I'd call that a "soundbite" rather than a "proverb". For something with a slightly longer history...
...which goes back at least to C17. In my experience, it's normally said disparagingly after a botched attempt to improve something, rather than as advice to start by using good raw materials.
putting rouge on a corpse.
Won't help a bit but will make it look more lively.
I think I can add some from the Jargon File:
Wave a dead chicken (to appease powers-that-be that everything that could be done has been done)
Add Bells and whistles (which doesn't imply the program is faulty but the action is rather useless)
If you like these, I recommend you browse the Jargon glossary, you'll find a plenty.
Papering over the cracks
From Merriam Webster:
to hide problems
We have to acknowledge these problems. We can't just paper over the cracks.
A recent example of the phrase from Belfast Telegraph:
Papering over cracks of flawed system in Northern Ireland won't work - we need to rip it up and start again
Bill Franke already mentioned putting lipstick on a pig, and “Ann” mentioned putting lipstick on a pig in an earlier answer and Kyle Pearson mentioned putting lipstick on a pig in another earlier answer. Also, Tom Au's suggestion of puffery, “that is puffing up the best features [...] and pointedly ignoring the worse ones” applies here.
Polishing a Turd (NAVY) Someone trying to make an idiot look like a genus. (Music Industry) The house band attempting to make the opening act singer sound good when they sing terrible. (Government) Promoting a person as having high moral management standards, when in fact they have no morality at all.