It's not just the UI that is bad, it's the whole user experience (UX).
It's best to start with this:
This whole screen is extra, it's not necessary, except, and this is where the problem lies, when you need to change almost any setting in the game
After that you get the login screen:

Yes, it is optional, but you need to actively turn it off, also without it i don't know if you can go silent or offline
And now, finally we're in the game. Oh, no wait what is this:
Another menu, what's this one for? Well to choose what game to play, also this one is the worst.
Imagine being a new player and seeing a menu like this for the first time, the last two were at least understandable, this is just a mess, what is clickable, what does what, what does channel mean, what does station mean. It's a mess that no new player can understand at a glance.
Ok, we're in the game. Wait what, all the steps before this were actually unrelated to the game, only now have we actually gotten in the game menu.
This one actually looks good, seems intuitive, why couldn't it just be the one to show up when we start the game?
Now we diverge, most new players would choose the solo play first, let's check that out:
What's this? I want to play the game, i want to see what i bought, not some silly rankings. Yes, it's a competitive game, but rankings is not be-all and end-all of the game. (i honestly hope this screen is here because i finished the campaign)
If i ignore the rankings page, and assume that new players first see the white campaign, it's not bad from here on out. You choose the map you want to play, the enemy you want to race against and play. Personally i'd much more prefer all 3 ghosts at the same time, so with that the enemy pick option is skipped, though that is a useful screen in it's own right. I would just move it elsewhere so it doesn't intervene in the new player experience (my 2 cents here, the rest of the UI is awful would need a lot more then 2 cents to fix it).
Now back to the online mode, where we expect the player to spend most of his time
This one is also not that bad, i mean, there is not much you can do with the server list. Yes, it could be improved in several ways, mode filter is really bad, and the all modes button is a bit too hidden, echelon and levels mean nothing. All in all could be much worse, so it's good for what it is.
After all that, and we found a server on which we want to play, clicked join and we're finally in the game playing. Yay us.
Oh and remember this is one of the cleanest routes in maniaplanet. If you want to do anything advanced, good luck.
For comparison sake let's look at rainbow six siege UI:
I clicked once on the shortcut to enter the game, and it loaded me into the menu. From here it's one click on the blue button to play. Simple as that.
To enter a server in shootmania i had to click 9 times!
It's 2 click vs 9 clicks, anyone can see the difference.