I don't think the game is mediocre, I do think their acceptance of the client's shortcomings is mediocrity in motion. LoL the client isn't LoL the game, and it's clear they value the product they can broadcast and market over the waiting room.

There's been plenty of problems with the client actually disruptive to gameplay, like the champion skills sticking out over the masteries options, people who couldn't accept their games, the client using 90% CPU back in 2010, a store that couldn't remember if you were logged in for a while, the terrible notification management that led streamers to ignore notifications entirely, muting friends on accident removed from your friends list without any confirmation, the ability to spy on your opponents ahead of accepting a ranked match, the ability to know their runes and masteries ahead of lock-in so that you could tell where they'd lane, the inability to reconnect to a match if you got logged out of PVP.net (unless you knew where the executable for the in-game client was), the followup inability to connect to the login server if there was a queue which caused you to be unable to rejoin your game from the player client, the client-side authentication of mastery points leading to zero cooldown summoners, the ability to sell your runes for in-game gold, using AIR under the guise of being cross-platform while failing to release a cross-platform game client for several years after, clicking 'play again' kicking you into your custom game lobby, disconnecting from PVP.net removing you from a room and causing you to join a separate game queue when your room leader starts, staying connected to post-game lobbies even when you leave the screen (you can hear the chat still happening if you listen for it)

I'm sure there's more, but the point is the client's code quality issues have been known since before season 1 and it gets treated like a second-class citizen of the game.

It doesn't take that long or that many people to make an alternative. Plenty of people have risen to the challenge and get quickly suppressed by Riot once the client is popular, usually because their APIs are exposing information that shouldn't be getting exposed.

If those people can do it, a paid team can do it, it's simply not their priority and hasn't been for almost seven years.

Only recently are we hearing about a new champion select experience, even then still inside the same client architecture.