is CotC still good on Diana?

It'd be better than most alternatives. I think TL is still overall better, but if you want to go a tankier build, it's great since the entire nature of the spell encourages you to fly in to everyone, hence getting the largest shield possible in team fights. It'll absorb the first blow when yo inevitably go balls deep. For squishies, you shouldn't need TL to assassinate them, but it's a nice flavor bit to your combo. Really up to you. I'd go TL in lane, at least mid, but if you take her jg, to gank you have to R and E, so you'll guaranteed be constantly using it, and you tend to go tankier in the jungle, so definitely a great pick there. Top Diana I've done like twice in my life pre-keystone, so idk. I would think TL would be better. Help you snowball vs early game tanks by getting past their higher base defensiveness and fight them like they're regular squishies. Your passive and CDs make it so any lead you get should snowball you since no tank will be out-dueling a leading Diana.

i've only brought it on aram diana but it gives really nice bulk.

Stormraiders / TLD are better on her currently.

Not that it matters but I thought this said Darius and just had to get rid of big thing I wrote in response