Greg Universe is not weak. Greg is emotionally supportive, caring, making personal and financial sacrifices to support his son. He is poor, yet he does manage to sustain a small business. All the money he makes goes to Steven’s care, he doesn’t live with Steven even though he wants to because it’s in Steven’s best interest, he’ll drop whatever he’s doing whenever Steven needs help or advice, he is involved in Steven’s life and knows Steven’s friends and their families. These are strong, positive traits, not weaknesses.
They are strong traits associated with the traditional role of “mother.” Greg fills the stereotypical role of a single mother doing her best to support her child. He is not a stereotypical father. When people object to Greg’s characterization, they are not actually objecting to a “weak male character” but to the blurring of their own gendered expectations and values. Greg is not part of some feminist plot to portray men as weak because it’s not feminism that says his traits are weak or even specifically “feminine.” It’s not feminism that says men have to be financially successful, physically strong, assertive, dominant, stoic, or stereotypically “masculine” to be strong. Feminism points out how we conceptualize and gender those ideas and values, but it didn’t create them and certainly doesn’t endorse them. If you believe in equality yet think those “feminine” traits express weakness when a man embodies them while being strengths when applied to women, consider what that says about what you (perhaps subconsciously) think of women and equality. Consider who’s really looking down on caring, nurturing, supposedly less “masculine” men if you’re seeing characters like Greg as weak.
Men aren’t weak for sharing traits with women. Supposedly “feminine” traits aren’t weak. It’s sexism that boxes people into those roles and makes that value claim. That’s the whole point his existence challenges.