He has been a regular starter because of how often Varane and now Martinez have been injured. He was starting a lot throughout last season and he's started in 6 out of 10 games this season. Been subbed in, in 3 others as well. For how average/bad he is, he's playing way too many games. Lindelof gets away with mid performances very often with how little activity his thread has.
I do think Dalot has been good this season. He was shocking for Zaha's goal, but he's mostly done well. He isn't one of the players that's been a problem.