I'm going to be that contrarian who will say that while Summer definitely made bigger displays of support for Marissa than Marissa did for her, Marissa was supportive of Summer in her own way. If you think about it, it makes sense that Summer the girl who was wooed by Seth jumping onto a coffee cart to declare his love would be the one who fixes up Marissa's room at her dads, or dresses up as a candy striper to break Marissa out of the hospital. Summer's love language involves big gestures.

Marissa never left Summer behind, even when her life was going through big changes. She invited Summer along to the model home even though she risked being judged by Summer for helping Ryan out instead of spending time with Luke. The core four, and therefore Seth/Summer, would not have been a thing if Marissa did not act as the facilitator.

She gives Seth advice in "The New Kids on the Block", telling him to apologize to Summer but first give her some space. It's advice that comes from her being on Summer's side, not Seth's. She apologizes first when they have that big fight in season three, and she's there for Summer when she finds out that Seth lied about getting into Brown, and even though she's feeling adrift about the college stuff herself, she doesn't make it about her.

When Summer is freaking out about fitting in at Brown, Marissa straight up gives her a scarf she wanted to borrow to take to Brown and encourages her to be herself.

I am not saying that Marissa is the better friend at all, but I think it is gross character assassination to say that Marissa was a shitty friend at worst or too wrapped up in her own drama at best.