I want to give fashion advice on the first date
Why not get to know them first?
Fashion is the most important thing you can discuss
It is rude to only talk about clothing on a date. If you didn't bond on fashion before they might be turned off and you won't get a second date.
But I can't stop thinking about them wearing my favorite pair of pants.
Giving fashion advice can be helpful. But not when the person hasn't asked for it. You can talk about fashion on the date, but hold off on the advice.