Dunno, but since the OP said that a circle was also showing pixelated, I don't that as the issue, and, even if it were, if it were smooth in the app before exporting, it should be smooth after as well.
The only way I get jagged or pixelated edges with those conditions is if I zoom in on whatever the screen shot was taken of. For instance here is a picture of a circle -
- Round....
- aPic_circle.png (2.07 KiB) Viewed 2273 times
Here is that picture exported side by side with the original merely close to the same size...
- ...and round....
...and finally, here is that circle enlarged by comparison, which finally shows some pixelation...
- ...and round we go....
...so I am curious exactly what is going on in the OP's case to see if it can be duplicated.