Is there really important to learn CSS if i want to be a web designer?
Yes, it's quite important. But, look at it this way. Once you learn how to use CSS, it can save you a lot of time in making changes to a site, especially a site with many pages. With a large site, you can change font throughout the site by merely changing it in a CSS file, if you've used the font style in your CSS file instead of setting font on individual pages, i.e., by not using CSS. Does that make sense?
Well, here's a reference that explains the benefits of CSS better than me:
http://www.yourhtmlsource.com/stylesheets/introduction.htmlCSS hard to learn if I know HTML well?
I can't say I'm an expert on CSS, but I think a simple answer is that it may be about the same difficulty to learn as learning HTML. And, yes, knowing HTML well will help.