Here is the problem: actresses should be good at acting, but we tend to get confused and admire or hate their fashion sense. When really at the end of the day they aren't stylists/fashionistas, they're actresses--they act! I think this problem emerges from their numerous public appearances and current domination of magazine covers. What Tilda Swinton wore to the Oscars (love it or hate it) shouldn't really matter, as what she specializes in and is paid to do is act--which she does very well.
Also, while we bemoan at the slip ups on modern red carpet and admire the polished glamour of yesteryear, let us remember that in those days stars did not have the style freedom they possess today. The studio renamed and remade (dye jobs, hair cuts, plastic surgery, stylists, you name it) their stars into the shells we love and admire. Studios even primped their leading ladies and lads for events--bringing a team of hairdressers, make up artists and stylists to the actor or actress for every prevalent event (and there were fewer events too!). So while, the golden age might seem lovely, it wasn't very free nor is it really within the reach of stars today.
Now don't misunderstand my words (here is where I say this is just an opinion, as if it wasn't clear that was what typed by one set of hands is one young lady's opinion...), actresses may dabble and dare I say, even enjoy fashion. There are many actresses/film industry buffs who's fashion I admire (Sofia Coppola, Chloe Sevigny, Keira Knightely, etc), but these ladies should be admired for a separate interest in fashion. They are not actresses and therefore stylish, they are actresses AND happen to be stylish. Just as I am a business student who happens to like fashion (I have many artist friends who are not innately stylish--they pay more attention to their paintings than their clothes).
So, to all the actresses who feel they must dabble in clothing lines, I am sorry for leading you to believe that as an actress you should be talented at fashion and design. The fault is entirely my own, next time focus on your craft rather than fleeting public opinion.