Being B tier at a couple things isn't exactly a sexy niche. It works for the bard, who is ALSO a 9 level spellcaster. What is the monk's excuse?
Existing to spam stunning fist is kind of lame. I feel it is a bit too strong compared to other options and crowds out other uses (should probably be 1/round). Monk's really just need some kind of mid-fight resource recovery mechanic.
The monk at low level outshines the fighter. But falls behind at high level. Strangely, in my game the monk is again:
Top damage dealer
Best scout
And also act as the trap finder/removed and picks locks.
She holds her own against any opponent.
The monk works out fine for most tables. Only tables that rises to high levels (11+) where the fighter will start to out pace the monk have any trouble at all. The monk however, will retain its power over shutting down single target. In essence, the problem is almost a white room problem as the vast majority will see the monk working as intended. Remove the Paladin's smite ability and the same complaints will start to appear. The monk should have had a third attack at around level 14-15.
We hear similar complaints about rangers and people still play them and TCoE did almost nothing for them either. Again an other attack around level 14-15 would do a lot for the ranger. And still, most tables will not see the problem because they never rise to these levels.
There is nothing wrong for a class to see a shift in its role over the course of the levels and there is nothing wrong to be second best at a lot of things. See the bard, it promises a lot, but second best healer and poney trick... not even close in damage to warlocks and wizards. Heck, it damage cantrip only does a d4 and disadvantage on one attack fades in power the higher level you get as most high threat foes will have more than one attack or will be able to cast spells.
But you claim the bard is fine. I do the same fir the monk. We just added wisdom mod to ki.
Also, the monk pretty much assume two short rests between long rests. Do you have these or do you simply have one fight per day? With no short rests, the monk does not compete at all with long rest classes. It simply can't. And without 6-8 encounters per day, the long rests classes simply have too much going for them. They do not have to withhold their ressources in case of...
A lot if the monk's problem can be directly related to the level you play and the style of your table. I strictly enforce the 6-8 encounters per day and only allow 2 short rests between long rests. Allowing more simply gives too much power to short rests classes, especially warlocks but the monk too as it can use all his ki points in every fights.