Suiton will create a larger distance between Mei and Sakura and her Lava isn't a long range technique nor has it shown any kind of superb speed, so Sakura would most likely bounce back from the Suiton (which as you mentioned can't do much damage) and and then dodge the Suiton and continue her advance. Sakura w/ a powerful enough strike to the ground doesn't need to be anywhere near Mei's immediate location to do damage meanwhile Mei has to land Lava to win.
Suiton doesn't need to be fired at a horizontal, which would be the only way to create distance between them. Even still, Mei can use shunshin to follow Sakura in mid motion much like in these cases (
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)(
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) and (
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).
Anyway, my premise has Mei launch her mid air via water pillar (Suiton: Suijinchuu). If Sakura is hit with it, she isn't going to immediately recover her footing much
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(
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) where she even had a prior warning to what was going on, which allows Mei to hit her even when Sakura's on the ground, if Mei, for whatever reason, is not fast enough. Furthermore, the
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to either propel Sakura further into the air or smash her towards the ground near Mei where Mei can go on to coat Sakura in lava.
Overall Mei can easily stop Sakura's only two ways of attacking, getting close or punching the ground. On the other hand, Sakura can't stop Mei from overwhelming her with suiton and then dowsing her with youton.
Also don't forget the intel advantage. Once Mei sees Sakura in Byakogou mode, she'll assume Sakura has a similar way of fighting as Tsunade whereas Sakura won't know Mei's ninjutsu arsenal.