Please review our startup (Forever Fluffy)

That said, your questions are valid. Yes, people memorialize their pets, and many do it online. For many, their deaths are as affecting as losing a human relative, and humans have memorial sites.

How will this site reach those people? How many will actually want to use the service? And would they pay for something they could do for free on an ad-hoc basis on a different website?

This is just my gut talking, but they should probably pivot to provide an end-to-end, just-lost-a-pet wizard (NOT a native app, but a website). Step 1 connects you with a crematorium, step 2 lets you buy an urn, step 3 lets you send invites to friends/family for a service, step 4 lets you set up the memorial page.

I know that might sound ridiculous, but losing a pet can be expensive and time-consuming, just as it can be when you lose a human. People just want to be connected with inexpensive services quickly, and Forever Fluffy could take a very small cut for making the connection.