I mean, it isn't just What If... that there exist variants who scale to each other. Multiverse of Madness is another example of variants of the main characters being introduced where the scaling generally makes sense. When the Illuminati fought Scarlet Witch, she easily killed the weaker members such as Mister Fantastic and Captain Carter, but against Captain Marvel, she put up much more of a struggle and forced Wanda to get serious. Wanda also easily overpowered a variant of herself who hadn't awakened the same Chaos Magic as she had, which fits into the powerscaling. My point is that while some episodes may have stupid reasoning, like Proxima being able to overpower Thanos, from what's been seen in the What If...? series so far, I think it's generally consistent, and it wouldn't make sense for each of the variants to have drastically different power levels.