No he won't. She'll start getting advantageous frames by 50% onwards, when the lock begins on Mario.
But it gets complicated - the lock works when she has a +5 frame advantage or more. The higher the enemy's percent, the greater the likelihood that she'll get this advantage. This means she can execute another dtilt straight after without you being to throw your shield up.
The more decayed dtilt is however, the less of an advantage she'll have, which means if she plays the stock wrongly, she might think she's locking you when really you can shield the next hit. But say she decays it.... but starts using it at 80%, then she'll still gain frame advantages because Mario's at such a late percent meaning she can chain them or dsmash straight after truly with no possible counter except SDIing away.
I don't recommend the lock though personally. It's good and bad for different reasons. I simply dtilt > dsmash at the optimum percent... because the dtilt will become decayed, but the dsmash will add percent to compensate for this decay, so you get equal hitstun when you use that dtilt again the next time. So dtilt > dsmash will always string together. It's nice damage builder from a fast move.
That's not forgetting the high trip rate of the move as well, and the fact that if she saves dtilt for higher percents, she can start comboing into smashes, dash attacks, tilts and grabs especially.
