It means someone who is naive, gullible, or easily fooled.
The image of a "straight owl" implies someone who is clueless and doesn't see things clearly, making them an easy target for deception.
Here's an example:
* "He's so ullu seedha, he believed everything the salesman told him."
This phrase is often used humorously, but it can also be used in a more critical way to describe someone who is easily manipulated.