Use it! While you're likely to have critters in the mulch going after the pecan nut remains, you'll also get to observe the critters in the mulch going after the pecan nut remains.

Whether that's a negative or positive is a matter of perspective. For me, that's a positive. I like watching wildlife in my gardens.

If you can chop up the leaves, hulls, and nuts with your mower, it'll probably make it more homogenous, faster to decay, and easier to spread--and less likely that the leaves will blow in winds.

But if not, you can certainly spread full leaves.

Your other option is to pile it all up in a corner of your property somewhere where you don't have to stare at it and let it rot for a while. It'll happily decay on its own and turn into leaf compost (with the nuts and hulls in there as well), which you can then pull out and spread on the gardens. That can take a while--up to a year or two. And again, you can chop it up or not first, whatever you want, and add other garden trash in there to compost with it.

There's a reason we refer to leaves as brown gold. They'll help your gardens hold water, add resources (a little, anyway) to the soil, and return what the trees used to make the leaf back to the ground instead of throwing it away. They'll add organic material to the ground, too.