That’s a very good point - he does, doesn’t he?
I think, generally, it’s because of certain aspects of his character, which are helpful in making friends.
For example, he’s curious without being nosy. He’s interested in what people do, and likes to find out about them.
He is very fair and honest, so that people can trust him, and he likes to make sure that people are treated fairly, and that if there is something wrong he will try to put it right.
No matter what goes wrong, or how much trouble he gets into, he remains cheerful, and tries to make sure that his friends are happy too.
Yes, he has quite a few friends: we see him meet new people and make friends with them in many of the books. However, it’s not the quantity of friends that really matters, it’s the quality of the friendship.
We see a small group of people over and over again – the Captain, Calculus, the Detectives – who are obviously close friends.
We also see that he remains friends even with people who can be irritating, such as Castafiore, Jolyon Wagg and Abdullah: that’s very important too, as everyone can find other people, even people close to them irritating at times. It’s because Tintin can stay friends with people like that, that we know he must be good at making friends.
Update: After having written the above
I found a link to just this very subject of what is important in making friends!