Someone asked this on HiNative and got this answer:
好き、はノーマル スキ、はコミカルもしくは少し冗談めいた表現です
I'm inclined to agree with the native, but I don't think it's necessarily always meant as a "joke". I'd say it's stylistically informal, in a way that's a little playful, or less "serious".
PS: I feel like this is one of those things native speakers might not fully agree on. Like, you'd ask 10 different people and get 10 slightly different answers.
Here's my vague impression of how katakana is often used for regular Japanese words - though it's based only on stuff I've read, so it may be wrong, in which case please correct me: it's a lot like the various formatting quirks used by speakers of Western languages when trying to give particular emphasis to something, whether for humour, for dramatic effect, for extra weight in an argument, or any number of nuances depending on context. Like writing "Ewww, that's OUT of the question!" or "She does that e.v.e.r.y single time!" or "Gee, wearing all black, that's so oRiGiNaL!!1! :PP" or "I totally didn't see that one coming."
Agree with responses, largely a stylistic decision or for emphasis. As for how or why it's being emphasized I think it's like everything else with Japanese: based on context.
