Biggest draws for visitors are food and drink shops, so if you have a lot of them clustered in the middle that's normal to have your visitors clustered there as well. It's one of the main reasons why food courts in PZ are bad; individual and paired shops every habitat or two work a lot better.
Also, where are your most popular animals? You want to have the animals with the highest attraction rating at the outskirts of the zoo (in all directions) to draw people outward and keep the more boring animals central.
If you're still expanding the zoo, it's also normal to have a glut of visitors in a specific location or two when the visitor count first spikes, because of how visitor movement happens (they flow like water, basically). It's not unusual even to have a year or two of rather unhappy visitors because of this as it makes the lines for shops where that big clump of visitors is exceedingly long. They will spread out eventually if you've spread out your shops appropriately, but can take several game years.
My tactic to avoid that clumping of visitors as the zoo grows is to set a visitor limit in options when I first start the zoo. I set it to 500 and raise it 500 at a time as the zoo grows...it keeps the massive clumping from ever occuring because I'm controlling the rate at which there is growth in visitor count. We can set that limit in all game modes now (except career scenerios).
Edit to add: And yes you can use coolers on paths. Personally I don't consider visitors worth the effort or expense, as it's easy enough to keep them happy regardless of their temperature comfort level...they should dress more appropriately imo...but you can if you want. I do air condition (or heat) buildings though, because...well, it just makes sense.
