If you've connected with Molly I'd keep her, but I personally would move her out.
I like cat villagers. I think Molly is cute, but I like Lolly better!
I've got a cute normal at my campsite too and planning to invite her over even though I already have 3 normals so I will have 4 (not kicking any of my normals out) ¯\_(ツ)_/¯ It really depends on ur preference. They are both cute.
I love Molly so I certainly woud not kick her out. Plus, I usually prefer to keep my villagers (I mean, if I like them) than to change them for others, unless I really love the new one much more. I always feel bad kicking someone out, and I grow attached to my villagers, so I try to keep the changes at a minimum.
I like Lolly much more so i would kick Molly out for her if i was in this situation. You already think about doing the same so maybe you like Lolly more, too?
Despite the fact that cats are my favorite animals irl, I find Lolly's design a bit boring and she lived in my second NL town for quite a while.
Also, besides Olivia, I find the cats who lack noses a tad bit unsettling for some reason.
I prefer Molly's overall design but it seems you're leaning more towards having kick her out so if you prefer Lolly, you should go for it.