I see in a way that top tiers most of the time bring somewhat of a fun in the game...
Tbh that depends on design. Thing is, NRS have a history of creating two types of "bad" top-tiers:
1) Ones that have a few strong moves that are universal enough that you can't go wrong with them. "OP" happens when said character doesn't have serious downsides and can basically spam his way to win (F23 EVO anyone?) regardless of what his opponent could do if he wasn't too busy being controlled so hard. Characters such as these hardly make the game more fun. People will use them to win, but that's about it.
2) Diverse "can do it all" characters. Now, these may be fun to watch for a while, but when it comes to NRS games, a lot of their less conventional characters (or occasional specialist that isn't following current game's trend) have at least one serious weakness. And, so happens that weaknesses can be easily exploited by having just right hard-countering move. Such characters tend to quickly make competitive list less diverse simply because how many characters they can counter. Think MMH who could zone many of CC characters out while completely destroying zoning of many others.
So, it boils down to the fact that in NRS games, "everyone is OP" works poorly because large portions of the roster get hard countered and cannot play their game against other seemingly equally strong characters that just happen to have a right tool of two.
There are game where "everyone is OP" kind of works. KI characters, for example, are absolutely bonkers if you compare them to characters in other FGs outside of respective games contexts. But at least basic mech of the game makes sure you can always do at least something and have a chance to get your own game going - no matter how insane opponent's offence is, it won't neuter free combo breakers; no matter what gap-free pressure and plus frames opponent has, it's punishable on a read; there's almost always some kind of invulnerable move or armor you can use knowing opponent's options in hes current position.
So maybe NRS games could work too if they had better and, importantly,
more interactive defensive options?