I think the principle is that you should separate your role on the Committee from your personal views. There is nothing, to my mind, that sending a tweet achieves to undermine this. It is true that Chris Bryant recused himself for, essentially, similar reasons, and in that sense perhaps Harman could have followed suit; but, given that the Committee as a whole agreed that she should be chair, and given that anyway Johnson's supporters would do doubt have found some other reason to object to any other Chair, it feels more like an excuse to undermine the work of the Committee because it suits some to protect Johnson, than a genuine concern.
As I've outlined, there are multiple objective questions that can be reasonably asked and assessed. I'm sure you'll read the report, when it is published, carefully to assess the soundness of the conclusions reached.