It's not just than Erza got more feats or better feats, it's also (more importantly) that Erza got feats from which we can make direct comparisons to Mira's feats, and hence make stat comparisons. And to be fair to Mira, I even excluded parts where Erza used POF, like the infamous "she's Erza moment" in Tartarus, hearing Jellal's 'voice' and using guild/tree power against Azuma, and Erza slashing Sema with broken bones.
Being MC isn't the end all. Side characters like Gildarts, Laxus and Jellal have shown feats from which we could draw direct comparisons to Erza, and they have shown better feats (in the few occasions they're allowed to shine)
Even the narrative doesn't support "Mira is Erza's rival" anymore. That's gone the same way that Natsu's rivalry with Gray and Gajeel has gone. Laxus and Jellal is probably the only rivalry in which some credible cases/arguments are there that still can be made, and that's still afloat only because they're both side characters and equally irrelevant in Mashima's eyes lmao