It wouldn't be used the same way. If you were to translate I think of a girl, it would be Jag tänker på en flicka. If you wanted to ask someone what they thought of the girl, you would ask Vad tycker du om flickan? As you can see, you would only use the two words "tycka om" next to each other if it means "like"
I don't think it's a subordinate clause thing. I think it's based on context: "att tycka om" can mean "to think of/about" or "to like", but if you translate the sentence with it being "to like" it doesn't make sense. "I don't know what to like her." Since "om" is the preposition, translating it to "like" eliminates the preposition from the translation. Therefore it has to be "to think of/about" in this context.
It would make sense to see "att tycka" and "att tycka om" like two different verbs. When you "tycker" something it means you have an opinion about it, but when you "tycker om" something it means you like it.
Examples: Jag tycker du är söt. (I think you're cute.) Jag tycker om dig. (I like you.)
A small anecdote: It's not commonly used, but you could also say that you "tycker illa om" something, which would mean you dislike it. (though normally we'd say that we "ogillar", "gillar inte" or "tycker inte om")
Examples: Jag tycker illa om honom. (I dislike him.) Jag ogillar henne. (I dislike her.) Jag gillar inte honom. (I don't like him.) Jag tycker inte om henne. (I don't like her.)
Not exactly :).
tycker - think(s)
tycker inte - don't (doesn't) think
tycker om - like(s)
tycker inte om -don't (doesn't) like
Note that "om" is stressed in tycker om/tycker inte om above. If you stress "tycker" in the following sentence
Vad tycker om honom?
it means "What do you think about him?"