When Dobby first meets Harry, he mentions how much better life had vecome for house elves after the vanishing of Lord Voldemort.
“Ah, if Harry Potter only knew!” Dobby groaned, more tears dripping onto his ragged pillowcase. “If he knew what he means to us, to the lowly, the enslaved, we dregs of the magical world! Dobby remembers how it was when He-Who-Must-Not-Be-Named was at the height of his powers, sir! We house-elves were treated like vermin, sir! Of course, Dobby is still treated like that, sir,” he admitted, drying his face on the pillowcase.
“But mostly, sir, life has improved for my kind since you triumphed over He-Who-Must-Not-Be-Named. Harry Potter survived, and the Dark Lord’s power was broken, and it was a new dawn, sir, and Harry Potter shone like a beacon of hope for those of us who thought the Dark days would never end, sir. . . .
Harry Potter and the Chamber of Secrets, Chapter 10
However, in further books, it is seen that house elves are still treated very poorly. In Harry Potter and the Goblet of Fire, no one takes Hermione's initiative, S.P.E.W., seriously. High ranking ministry officials like Amos Diggory apparently consider it beneath them to address an elf by its name, as was the case when he addressed Winky. The general consensus, probably even among elves, seems to be that everyone prefers things to stay as they are.
Even Hagrid, who usually tries to ensure the well being of all magical creatures, dismisses Hermione's attempts to make him join S.P.E.W. Probably the only wizards who are show to be pro elf rights are Hermione, Dumbledore, and Arthur Weasley.
Dobby thought differently from others of his kind, so his perception of "better" would be different. It must be noted that Dobby was referring about the condition of other elves improving, not his own. Based on what we see, Dobby's expectations regarding treatment of house elves are far higher than most other elves, which means a significant improvement must have occurred.
In fact, when Voldemort returns to power, it seems as if his beliefs reduce the status of muggles, muggle borns and half bloods to the status of house elves and the like, without affecting house elves directly.
So, did the condition of elves really improve, to the extent that Dobby called it an improvement? In what way? And why, if wizards and elves alike didn't want things to change?