"Improvement" here becomes a metaphor for a lot more than HGTV-style home redecorating. It also applies to the characters who inhabit the houses that are undergoing this renovation. These houses often reflect their owners' personalities. For instance, Mr. Rushworth's "stupidly" designed house may be making a comment on its owner. Here's an example:
The situation of the house excluded the possibility of much prospect from any of the rooms [...] Henry Crawford was looking grave and shaking his head at the windows. Every room on the west front looked across a lawn to the beginning of the avenue immediately beyond tall iron palisades and gates. (9.9)
There are some really interesting things going on here. First off, the lack of a "prospect" refers to the fact that it's hard to see very far from the windows. The house has a limited perspective and view, which reflects Mr. Rushworth's inability to fully see what's going on with Henry and Maria. Also, we get a description of a view that contains iron gates. This implies that the house is a bit like a prison, which reflects how Maria views her current situation and will come to see her marriage to Rushworth. So there's some foreshadowing imagery, or imagery that gives us a hint of the future.
Of course, Rushworth wants nothing more than to "improve" his house and Henry wants nothing more than to do it. Improvement is also linked to themes of movement and activity. Henry likes "improving" things because he likes to be active and busy at all times. However, it seems that Henry likes improving for improving's sake, so to speak. In other words, he likes to be doing things, no matter the goal or end result. Thus, Henry's improvements don't really have a lot of substance behind them. Not all improvements are created equal, and the theme of home improvement is a way to reflect characters' attitudes and the flaws they have that need reforming.