Morning!
I fail to see what is 'unnice' about the second image?
It looks fine at 100% zoom. The extract has been greatly enlarged from that original, so it is no surprise that the quality is bad.
If you want to have the same quality in the whole map as you have in the room you first rendered, then you will have to export a very large map indeed.
...
This is a separate observation to the issue of size and resolution. The bitmap fills you are using are designed to be used in battlemaps that are usually exported at a much smaller size than the first map. They look good at that smaller size, which is closer to the second map above, but they start to pixelate if you enlarge them too much, which is beginning to happen in the first map.
This isn't a fault or a problem. Its just something that you need to be aware of