It doesn't personally bother me, so long as it remains in the minority of posts. Couple of reasons:

a) It often reminds me of some of the good boys I've lost, and of the good times we had. It's a bit bittersweet, but it doesn't have to be a mood kill. I acknowledge if you've lost a pet very recently, it'll hit a lot harder, but this is going to happen regardless as you process your grief.

b) I'd prefer to be part of a community that's not continually mood-checking each other. Life has ups and downs, and everyone is better off if the community can occasionally offer some consolation in the hard times. Expecting only happiness from everyone always seems like toxic positivity to me.

c) Offering a word of support can make me myself feel better. It can be a good deed for the day, and it'll make me feel closer to the community.

d) I don't think having a bit of a memento mori is necessarily bad. Being reminded that our time with our companions is limited can also remind us to cherish that time all the more.

I do think there's a caveat, though. While loss is an inevitable part of animal companionship, it's very unfair if I only share my grief. If I enjoy the community and want to keep it healthy, then I should be proactive in generally posting uplifting content before I ever consider asking the community for their sympathy.

Also, personally, if I'm going to share a memorial post, I think it'd be best if it can be a celebration of my pet's life, and show how they had a good, and happy one.