What's wrong with this indoor plant?

[Existentialallday]

I repotted it a couple of months ago and since then the number of these mottled leaves has increased. But I can see tiny spurts of growth, so I haven't killed it? Or have I?

Oh gosh - sympathies. It doesn't look like you've overwatered it but perhaps fungal leaf spot. You can get stuff to treat it from garden centres. Keep it away from your other plants. This might help:

heyrooted.com/blogs/common-issues/how-to-treat-leaf-spot

Good luck