Is 헤타리아 a 야오이 anime/manga?

No, it's not.
It's just a parody of historical events and politics between countries. As the matter of fact, Hetalia's main demographic/targetted audience isn't even girls.
The original 망가 is a Seinen 망가 while 헤타리아 World Stars is Shounen, both targetted towards male readers.

I've noticed that most of the people who are claiming it's shounen-ai/yaoi are either people who haven't watched 또는 read 헤타리아 and simply assumed its genres 또는 they're the same teenage 야오이 fangirls who are also claiming that Haikyuu and Kuroko no Basket are shounen-ai. Not really that reliable, in my opinion.

Then there are the 팬 who think 헤타리아 /should/ be categorized as shounen-ai. However, thinking that a certain 아니메 and/or 망가 should be considered as shounen-ai doesn't make it shounen-ai. At the end of the day, 헤타리아 is still just historical satire.

No lol.