Ryzen does benefit from higher memory speeds (to a certain degree, I certainly wouldn't waste my money on a 4000Mhz kit for an R5 3600), and you can definitely get (assuming we weren't in the midst of a pandemic) a good high speed kit for reasonable prices...
I'm not sure how much you'd notice the difference, for example, between C16 and C18 on a daily basis, but I guess that depends on the workload.
It's really a budget and purpose question, how much can you spend? what are you working with? (I think 3600 C18 is good on a tighter budget, but you could go for C16, or tighten the timings yourself)
I wouldn't spend money on anything better than 3600 CL16 though, unless you decide on a more powerful CPU (and therefore a higher-end system in general).
*I would make sure that whatever you get is compatible/ tested for Ryzen, just for peace of mind. you can run the model number here:
https://www.amd.com/en/products/ryzen-compatible-memory/ready-for-3rd-gen-amd-ryzen
You should also run the memory by the motherboard manufacturer's Qualified Vendors List, to make sure it's been tested.
**I'll note that most of what I said here mostly comes from watching Linus, Gamers Nexus, Paul's Hardware and others.... not from personal experience.