Question: When a witch or wizard transforms into an animal, we know they can still think and listen, like Pettigrew did when living as a pet. But according to Sirius it can alter one's emotional state. Could the simplicity of an animal's emotions be a benefit to someone trying to defend against Legilimecy? 

In PoA, Sirius Black explains he was able to escape the dementors by transforming into a dog.

> 
> "They feel their way toward people by sensing their emotions... They
>  could tell that my feelings were less human, less complex when I was a
>  dog...
> 
> 
> I slipped past them as a dog... It's so much harder for them to sense
>  animal emotions that they were confused."
> 
> 
> 

This implies a lack of emotional complexity might be used as a defensive measure against certain magical creatures. In OotP Harry learns from Snape that Occlumency is the ability to close oneself off from certain emotions and that it protects against Legilimency.

> 
> "[Legilimency] is the ability to extract feelings and memories from another
>  person's mind...
> 
> 
> ...The Dark Lord, for instance, almost always knows when somebody is lying to him. Only those skilled at Occlumency are able to shut down those feelings and memories which contradict the lie."
> 
> 
> 

He implies that being in a state of high emotion makes someone an easier target for Legilimency. During these lessons, Snape seems to expect that Harry's defensive Occlumency will mostly consist of mental and emotional control, and the extent to which magic is necessary for this is unclear. Since an Animagus likely can't cast spells while transformed, would they be an easier to 'read' via Legilimency? Or does the simplicity of an animal's emotional state make them harder to examine?

I realize there isn't a definite answer in canon so I am looking for a plausible answer with support from canon.

Answer: 
It's not specifically established how targeting works with Legilimency. All the instances I've found involved line-of-sight contact with the target, with the exception of Voldemort and Harry being the only remote-targeting instances.

I would guess that an Animagus when transformed would be unlikely to be targeted, unless the Legilimen suspects the animal to be a transformed wizard; it's good camouflage. If a transformed Animagus was targeted, I would think that while current thoughts and emotions may be harder to read, memories would still be intact and available.

In the case of Harry and Voldemort, they were connected and able to read each other from afar. I don't think if Harry would have become an Animagus it would have made any difference.
It would probably be easier, as when the wizard turns into an animal they become more like the animal, and since animals aren't as intelligent as humans their mind would be less powerful, like the animal. Now, I'm not saying that animaguses turn into that animal when transforming but according to sirius black it does change the state of their brain.