Language is hardly ever direct or exactly literal. Sometimes when we don't know something, the only place to start is literal.
'Hot tropical girl' is not a set phrase, that is, the term does not evoke something more than the combination (after cultural inferences have been made).
It would be too mechanical (or rather in very very uncommon circumstances) for it to mean a female human of age under 18 whose skin temperature is above 102 F and was born between 23.5 N and 23.5 S longitude, bleep bloop bleep, computation terminated.
'Hot girl' here mostly means a younger woman who is attractive. 'Hot tropical girl' is the same but showing tropical features like medium skin color and few clothes. What 'tropical' is exactly is just as vague as 'hot' or 'girl', it has a broad enough range that it depends on the person who used it, the audience, etc. Pinpointing it to individuals with particular nationalities with particular names might be misleading; you might be unnecessarily leaving out Mildred the Glaswegian.
The entire phrase is not really a 'thing' (that's what it means to be a set phrase). It's just this one guy who came up with that phrase (others may easily do so too, but it's not a 'thing'). It's the difference between 'white house' (a house that is white) and the White House (a house that is white but is capitalized and other stuff).
I can easily anticipate that your next immediate question is about the term 'Viking beef cake' (eg Liam Hemsworth as Thor). He is (arguably) not a sugary floury bakery item made out of cow muscle who invades Northern Europe from Scandinavia circa 900 AD. We know this because Liam is from Australia.