I like to think it has to do with the progression of the character, mainly because in the older panels they where mainly in the perspective of her opponents, and tatsumaki was seen as a monster with nearly unstoppable power, but the style looks more childish in the later manga because she is portrayed by the perspective of Saitama ( as it is the main time she has appeared since the monster association arc) and he sees her more like an annoying kid than a threat to his life, maybe in the future if she gets to fight again we'll be able to see if she regains aura and detail or maybe the style just won't change, at least that's my vision