> I thought kimonos are thicker than yutakas?

I think so.

> Orikasa Ai's kimino/yutaka seemed to be made of heavy material.

It looked like a cotton/something mix.  But I did ask her point-blank,
and she said it was a yukata.

> What is the difference between a kimono and a yutaka?

Kimono are much more formal than yukata.  Think of the difference between
a sports jacket or smoking jacket and a tuxedo jacket.

Jeff