Yeah they are. Mostly. Well, it gets weirdly complicated.

Top level. Yes. The isolate has some genotype, which determines its phenotype. The one we're interested in is the antigenic profile, which is to say those "things" that our immune system sees. Some are lipids, some are proteins, some sugars, etc. Our immune system responds in a certain way against a pathogen with one particular antigenic profile, and a somewhat different way against a different isolate of that same species if it has a different antigenic profile. Different isolates are of the same serotype if our immune system responds to them in the same way.

I say mostly, because our human genotype also comes into it. And some species have ways for individuals to hyper-mutate or randomly change parts of their antigenic profile. So serotype doesn't correlate with anything like virulence or antibiotic susceptibility, and is basically meaningless. There are also some species who don't cause an immune response. So they don't really have a serotype. And other species who all cause the same immune reaction, and don't really have serotypes either.

And here's the fun bit. The word "serotype" kind of means different things depending on the species. It's basically defined by the field. Decided at conferences. So there's no actual single true answer to your question. Which is why I say it gets weirdly complicated.