'Borderline' can be a noun, adjective or adverb. Here it is an adverb, meaning 'almost'. It is telling us 'how' inappropriate something is... 🙂

It's confusing because you're right, "borderline" is typically going to be used as an adjective (maybe a noun) but here they are using it very colloquially. Very casual and fluent language. They're actually using "borderline" in this example more like an Adverb or degree, sometimes i call these "intensifiers" - so there could be something "extremely inappropriate" , "very inappropriate", "kinda inappropriate" , "a bit inappropriate" and finally "borderline inappropriate". So if someone says that this is "borderline ___adj___" they mean it is ALMOST, but NOT QUITE "adj" examples: This is borderline spicy - (this is almost but not quite spicy) That movie was . . . borderline funny - (almost but not quite funny) "How was your date?" , "It was on the borderline" (here i am using "borderline" as a noun but i mean it was ALLLLLMOST bad, so it can be used in a very similar way to the "adverb/intensifier" version.