I guess it's not about the fact he's clingy...that's kinda cute sometimes. It's about the fact that he's controlling..
He just wants full attention. He hasn't gone to controlling what he eats or breathes. Its still fine. And the guy doesn't mind.
Controlling who your partner should meet is also wrong. Well I guess it can be different for everyone
If it was friends that he was properly introduced, yet still controlled it, then yeah that's controlling. The cupid thing, and worse the other two who are honestly quite annoying is different.
Taejun apparently LIKES the attention. So people are annoyed with Hongyeon for being obsessively clingy, see that? Taejun didn't mind.