so as an example, lets compare a lvl 3 monk to a Ogre, a nice solid "big hitter" at CR2. The ogre does 13 damage vs the monk's reduction of 3 (dex) + 3 lev + 1d10 = ~11.5. The rogue's uncanny dodge would reduce about 6.5.
So a monk with decent rolls can almost entirely soak the ogre's attacks (and that's before things like using the dodge action). That's a big change from what they had before.
Now is that really a problem? Personally I don't think so. Ultimately this new monk is VERY good against single targets. Aka if you have the scenario where your in a tight tunnel and its one melee creature versus your front line, the monk might actually be your best bet for that position. But their ability weakens dramatically versus 2+ creatures, in which case your high AC fighters or raging barbs tend to do much better.
So to me this new ability:
1) Provides a general amount of durability to the monk, which was sorely needed.
2) Gives them a fun niche as the "1 v 1 tank". You will get monk players who will enjoy that standoff with the one big monster, and soaking round after round of damage.
And I think that's all great. Again this ability gives monks a focused niche, but it doesn't overshadow other fighters in many other common combat scenarios.