Dodging is not the only way to deal with an attack. You can also block it, which was what Mirajane was trying to do to Juliet's attack. She had her arms up to block and everything. She just didn't expect to take this much damage from the attack. She underestimated them. Now, against Heine's attack that followed Mirajane was simply too slow to react in any way. In that instance, she couldn't dodge or block because the attack was simply too fast for her to move. The attack was faster than her reaction time. Then against Juliet's kick afterwards, Mirajane was once again too slow to react, much like she was too slow to react when Freed punched her in the pages I posted above. I'm reading back on Mirajane's fights with Freed and Azuma. Satan Soul was never a speedster that zipped around dodging everything. The only time Mirajane was truly fast was in the anime fillers.
This isn't a case of Mirajane being nerfed. This is a case of Juliet and Heine being strong opponents for Mirajane, and her underestimating them. Don't just take my word for it. Take hers. Mirajane says flat out that she considers these 2 to be strong and that she underestimated them. At the end of the day, Juliet and Heine were opponents that forced Mirajane to use Alegria, her strongest takeover and the near-exhaustion of her magic power.
Now ask yourself how this stacks up to Erza's performance throughout this arc. Irene basically enchanted Neinhart to be so powerful that he was immune to Brandish's magic to fight Erza. Natsu lolstomped Neinhart, and then went enraged with END mode. Erza stopped Natsu and Gray dead in their tracks bare handed, then went on to fight Irene. Irene smashed all of Erza's bones and then pulled an asteroid from outerspace. Erza, with all her bones smashed, cracked the meteor open like an egg and then cut Irene out of her dragon form. Even exhausted, Erza (again with all her bones smashed) still got up to headbutt Irene. Does it make me a Mirajane hater to put Erza above Mirajane? Lol. I just simply don't believe that Mirajane can go around popping opponents far above her level in strength with macro when the manga literally never shows this happening, even when the situation would be seriously advantageous for her to do so.