I'm not sure if we call Croatia the underdog team because in the 2018 World Cup they were in the final with France
But if we talk about Morocco, yes they deserve to be called the underdog team especially if we look at the team that has been defeated
Morocco real challenge is against France if they can win then there is potential for them to win.
I don't agree to call Croatia underdog hahahaha, it's not a top tier squad like Brazil, Argentina, France, Germany, Spain and Italy, but it's a great team.
Their history is not old like countries above, their first participation was 1998 and so far they achieve: 3rd place in 1998 and 2nd in 2018.
Modrić, Periić and Kovacic are probably the best trio of midfields of the World Cup.
But Morocco I agree, it's really the surprise of this World Cup, but in my opinion France is going to win without worries, I believe in France in the finals, but Croatia and Argentina I don't have a strong opinion, both are capable and both are playing at the same level
Even though you could say Croatia is not an underdog, among the big countries you mentioned, Croatia is not so favored by most people. But now they can compete and play very well and solidly, even their game is much better than the top countries.
and for morocco everything was unexpected and no one expected that they would qualify for the semifinals and morocco were the underdogs and in the semifinals they would face france which in my opinion was not easy. And Didier Deschamps must have read the Moroccan way of playing and be wary of Morocco's counter-attacks, it's a game towards the final and both are good and have the chance to make it to the final. But France are a great team and all the players have quality and Morocco have a very strong defense and also rely on deadly counter-attacks.