How do decorations influence the number of visitors?

Decorations definately improve visitors, but paths add tons of them at a low cost.

Vistitors really freaking like paths.

Before I filled that island with paths I had 90 visitors. Decorations will cost you way more and get you maybe a visitor or two per object. Buy decorations because you want, not because they add visitors.

Note that there are many other ways to get visitors, they're not just from decorations. Also don't put much stock in the specific suggestion the Vistiors button gives you, it appears to be random; you can get the "More Islands" tip when you have the maximum amount of islands for example.

Also, apparently the visitor boost you get from building all these paths is temporary. After installing all of these paths visitors eventually decline down to what they were before you built the paths.

Generally, don't even worry about visitors; the number does not impact your cash flow. You earn cash because of your dragon's earn rate even if you have 0 visitors; it's magic.

Visitors are a fun way to see "how well" your park is doing but they're a rough metric with no in-game benefit whatsoever.