The last thing Lawrie said before the Tim-Miranda argument was "pretty awful, actually" - referring to Nicola's performance as Shepherd Boy. Which is definitely, positively, Lawrie being an ass. Nicola wasn't awful at all.

So maybe when Miranda put Esther on the spot and she didn't have time to think and get nervous - she came out with what she thought at that instant (especially with Nicola being her Best Friend). She wouldn't necessarily think Lawrie was an ass all the time.