The dialogue does not explicitly state why Lisa is not ready yet. The dialogue snippet provided mainly focuses on the discussion about clothing choices for a party.
Clive's disapproval isn't directly stated, but it is implied. Clive states, 'Well, I was going to, but seeing as you look so smart...' This suggests he is reconsidering his initial plan based on Lisa's appearance, indicating he might prefer a different standard of dress for the party.
The dialogue does not say that Lisa *can't* wear her black trousers and velvet top. It simply implies that Clive would prefer she wear something else.
Clive decides not to wear his jeans because he acknowledges that Lisa looks smart. This implies he feels that wearing jeans would be inappropriate or less formal compared to what Lisa is wearing.