Botanic as a station name did not seem too unnatural it has to be said. Adjectives do stand on their own like that in some applications.
Sounds odd to me in a way "Botanic Gardens" does not.
I have had a comment on Twitter that many people now describe going there as "going up the Baltic" - it does sound less odd with "the" (well, classic Scouse "de", I guess), but then that is the norm in Liverpool anyway - people shop at such supermarkets as Deasda, Dealdi etc, just as people elsewhere shop at "Tesco's" despite the possessive never having appeared in their branding. (J. Sainsbury of course added it to their branding, recognising that that was what people called them anyway, as did F. W. Woolworth and Company).
So anyway, all that waffle aside, "The Baltic" sounds a bit less odd than just "Baltic", but "Liverpool Baltic" not only sounds odd but also breaks Merseyrail's existing naming convention. There are already two other stations starting "The" so it is not ground breaking.