Can I respond to comments in Software Center as app developer?

There is no way to do that through the Ubuntu Software Center UI or through apps.ubuntu.com. However, you can use this workaround:

1. Submit a query to the reviews API for your app. It will look like the one below. Just enter this as URL to the browser and you'll get the data.

   http://reviews.ubuntu.com/reviews/api/1.0/reviews/filter/$LANG/ubuntu/$SERIES/$VERSION/$PACKAGE/

   Note: for commercial apps, 'Ubuntu' is not the origin, as they are published in PPAs. Use the URL scheme below for them:

   https://reviews.ubuntu.com/reviews/api/1.0/reviews/filter/any/any/any/any/$PACKAGE/

   This will get you reviews for a package. Note that most fields here can be replaced with any. For example, here are reviews of Warsaw written for 11.10.

2. Once you've got the data on your browser, look for the reviewer_username value on the review you want to reply to. This will correspond to the Launchpad ID of the reviewer.

3. You can then go to https://launchpad.net/~$REVIEWER_USERNAME (substituting $REVIEWER_USERNAME by the right value) and contact the reviewer through their e-mail. If they've set their e-mail to private, you can use the Launchpad contact form to get in touch with them directly.

It might be a bit convoluted, but it should get you what you need.