I found a good AP Evelynn jungle build

I think abysmal or wits end would work better over lb, especially if your team comp has a fair bit of ap since your lacking Mr.

Eve is fine late game. The problem is the first 10 minutes.

By the way, if going AP Eve, I highly suggest Rylais+Liandries (and for a tanky Eve either of those will likely outperform Luden's Echo, although I admit not testing it on Eve yet).

Why if you are not building much AP?

That is the general problem for AP Eve, most AP itemization scales with more AP so building any pure defense items cuts down on your offensive power even more so than with AD builds because are so strong as even without scaling multipliers.

Full AP Eve is great as long as neither 5v5 nor even 4v4 fights never happen though, so it is something to consider if the relative team comps already make winning such fights impossible (if not like at least 10k gold ahead before 20 minutes) regardless as it is very strong for making picks.