Build in single player, most servers suck anyways and no one even builds in them.
As for your building problem, it's hard to be appreciated for your work when rating is usually done with the first impression. If you spent days to build something truly awesome and took a couple bad pics in your haste, you will get a low score. It's also difficult when standards are stupidly high. Your building skills will be compared USSR building skills (example) which produce about the most impressive builds out there (details in details inside more details, the loving cracks in the walls are finely tuned details for crying out loud). I've seen some impressive builds blown out of the water because they were overshadowed by some other build.
Of course, this is a two problem conundrum, another possible problem is that your builds simply don't shape up to be anything special to begin with. Great builds require huge amounts of time to create, most of this time is spent before the actual building process. That time is practice and an assembly of ideas you get from building other things. You might even have experience outside Blockland, building with real lego/megablock bricks. Building could be easily compared to art, you aren't born an artist, you will have to draw before you can start painting.
As you get better and more intricate in your builds, you will also be encouraged to try different things in your builds. Instead of using 6 x 6 x 5 walls in your builds, you might start building walls with introduced patterns, creating carpets on the floors, making eye-catching objects etc.
The point is, you will become creative in your work, and that's when truly amazing builds are born, small or large.