Regardless of her reasons and politics, do you agree with her conclusion that Johnson's mismanagement of brexit is a much bigger crime than the expensive curtains?

No! (in answer to the direct question)

I don't consider that Boris has mismanaged or mis-sold Brexit. He had a poor starting position because of the position left by Theresa May. I still believe that Brexit will have been a positive move for the country in general.
As for his legacy, yes, he is more likely to be remembered for Brexit than for the refurbishment of 11 Downing Street. He is also more likely to be remembered for his handling of Covid.
In the 'great scheme of things' how the refurbishment was paid for is a minor matter which should not distract from the more important issues facing the country.
But of course, there are elections pending (particularly in Scotland and Wales) so jumping on this could be seen as the opportunism of the opposition.