Check your calibration, the song plays the same tempo/bpm throughout as long as you don't trigger one of yu's debuffs. The real challenge of the song is that unlike every other song, the pattern of the notes isn't consistent so while the BPM is the same, the number of notes and frequency you're expected to hit changes by the pattern.
If your calibration is good I would generally encourage critical or needle as your good luck charm freebie, but there is also strats around glass or even ritual, since Yu x Mei is the only song where ritual can only give you beneficial buffs. Theres also a cheese strat using retribution, where you take retribution as your freebie and then intentionally fail by spamming the beat key before the first note can line up, which grants you a couple free powerups. As long as you don't actually miss a note this won't interfere with a SS perfect run.
Personally I go for crit + needle for my first ones until you max out crit (with 8 cats eyes) and 5-8 needles so that you can quickly make Mei cum.