are you using raster based graphics? (png, jpeg, etc)
If you are then make sure you are using higher resolution graphics, so the pixels wont be so apparent.
If you are not, (if you know you are working with vector art) then check your preferences and see if Anti-alias is turned off. If it is off then just turn it on and the lines will get smoother.
Edit > preferences ☑ Anti-alias
Ibis Fernandez | (available for hire)
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Professional Animator, Filmmaker | Creator of the highest quality (modular) G2 rigs for cartoon animator and developer of Toon Titan and Puppet Producer
Author of Flash Animation and Cartooning: A Creative Guide
>>> be sure to check out http://toontitan.com for professional grade assets, templates, and custom tools for Cartoon Animator and more.