If you're exporting as PR4444 (which is default) - premultiply alpha should be set, otherwise, you can have that option turned off. Setting render quality to Best also helps. Both of those options can be accessed in the Export dialog under the Render tab (as well as most other options that override canvas settings.)
... other than that, I don't know why the export should be pixelating.
[screenshot from exported PR4444, premultiply alpha, best quality -- with a few
added emitter alterations for clarity.]