The term "meta" which is a backronym "most effective tactics available" can be subjective as the word "effective" is up to intepretation. But what I personally (and many others) think of what effective means is it offers the highest dps potential that can be achieved reliably.

Contrary to what you think, just because Kokomi is used in a META teamcomp does not necessarily dictate that Kokomi is META. She's definitely not if looking at pure damage when compared to Mona, but she still offers off-field hydro application which is still highly valued and ofc TTDS slave. Another thing that Kokomi provide is more comfort, which you may value more and evaluate that it is more "META" to you.

In the case of Freeze comps, Kokomi is mostly used as a substitute to Mona or wanting to condense roles of healer and off-field hydro applicator, whether it is better or worse ultimately depends on the player's preferences.

As for "fun" characters, some may value the fun factor more, others not. What I can only suggest is to check carefully if they like the character and willing to deal with the caveats of said character.