Is it considered bad to post your new web app here?

See all the "Show HN" posts. As long as you actually have something to show, and not just a landing page.

People do it quite often. Just prepend the title with "Show HN"

Examples: http://www.hnsearch.com/search#request/all&q=title%3Asho...

With a Show HN title and some openness to feedback+questions you should be fine. I think as long as you don't treat it as free advertising no one will complain.