This is a very astute comment, there's a lot of biological weirdness going on when you take a close look at Pokémon. Even the word "mammal" comes from the world "Mammary gland" which Pokémon don't seem to have (or need). And although there are egg-laying mammals, they still produce milk for their young (though not with nipples) and nurture them in a way that is antithetical to the behavior of most Pokémon.
Long story short- you're right, Meowth is being described as mammalian due to its superficial resemblance to cats and not because its biology actually lines up with what we know "mammals" to be. There isn't really a better word for them, unless I want to invent a bunch of fictional biological classes. And I seriously don't have time for that
(plus they'd all probably sound silly and made-up).
If you're curious about the basics of Pokémon biology, I do go into it in more detail in the full Pokémon anatomy textbook.
It's not currently available yet, but Patreon
supporters get a 65 page excerpt that (among other things) describes Pokémon eggs in more detail