If it's just for fun, it will be fine because sometimes we also want to bet on something unknown out of curiosity.
And if the statistics of the two teams are not too far apart and we have difficulty choosing the team, we could choose the underdog because sometimes the underdog can turn things around and in the end, they win.
I once casually chose a team from two teams that would compete and I didn't know either of them but I wanted to place a bet at that time and by chance, I won

Yeah, any clubs that will play in FIFA Club World Cup are best of the best club of each regions because they've a selection first. Since we're not really know with the club's power, there's a good chance of the underdog beat the favorite since it's got underestimated by many people.
It's better than bet on underdog where you're really know the league, example Nottingham Forrest vs Manchester City, I don't think any single person will bet Nottingham Forrest lol.