I still find the idea that it's "dumping on" him in any way to be totally bizarre. If someone else said that Superman was weak, that would be dumping on him, sure. But if Superman himself says "you're stronger than I am," if he's humble and selfless and praises others instead of himself, that is something to admire about Superman. It's portraying him in a positive light, not a negative one, because it shows how kind and giving he is, how dedicated he is to building others up instead of feeding his own ego.
Honestly, it's just basic, everyday courtesy and civility to praise others' worth above your own. That's why we say "The pleasure is all mine" when we meet someone -- it's a way of praising their worth while downplaying your own, of saying they're more of a pleasure to know than you are. Only a jerk would say "I'm better than you" to someone's face, so of course Superman isn't going to do that. He was raised to be polite and kind to other people, so he will praise them at his own expense rather than the reverse. It doesn't mean he really is weak; it just means he's selfless and respectful of others.