I liked Monika's Route but also didn't like it. It's weird.
It's made apparent from the beginning of the mod that the game serves as a psuedo-sequel to the original game. Although you go through the original game's Act 1 again for each route, you have a short scene with Monika upon first booting up the game that clarifies that she still remembers what happened in the original game. So she has those memories all throughout.
Monika's route is only unlocked if you get at least the good ending on all other routes, or if you enter the Konami Code on the title screen. So her route serves as a "Last Story" of sorts.
Once either of those conditions are met, you can start a new game and the first line of dialogue will offer a branch.
"-just like any other" is the line from the original game that will let you go down Sayori, Natsuki, or Yuri's route again if you want, while "-but something feels different" will allow you to start Monika's route.
You will play Act 1 again but it's different. As if to imply the other three girls retained their experiences with you from their routes despite the game being reset, Sayori seems to have her depression under control and seems genuinely happy (for example, at the very start, instead of catching up to you in a rush, she's on time and asks if you're proud of her for getting up early and being on time lately), Natsuki is a bit calmer and less-standoffish, and Yuri doesn't stutter as much and seems more confident in herself. You still play the poem game, but it doesn't matter what you pick as Sayori, Natsuki, and Yuri will always give a neutral response even if every word you pick is one they like. Monika will resonate more than usual though. There are other things like it will use Purist Mod's original soundtrack even in Act 1 (in the other routes, it still uses the original DDLC OST for the the original game's Act 1), and Monika being the one to diffuse Natsuki and Yuri's argument.
One other little detail is that sometimes Monika will deviate from her normal, happy facial expressions in Act 1 even while others are speaking, looking at you with displeasure or staring off to the side dejectedly.
This was probably my favourite part of the route, because it sort of gave me the feeling of playing the original game's Act 2 again. Not as horror-centric and anxiety inducing, obviously, but it was fun spotting the little differences. It was also refreshing because the other routes leave Act 1 completely unchanged, and while I played through it completely for Sayori's route to reacquaint myself with the game, I basically skipped it for the other two. So it was nice to give it's existence real value here.
However, it's when you get to the festival preparation weekend that it starts to go into the completely original stuff. Sayori doesn't go home early and suggests that Natsuki and Yuri come to her house so they can all work together on their preparations, leaving you with the chance to have Monika come to your house. In the middle of her visit, she will have an outburst and tell MC that nothing is real and not only is it a game, but a mod of said game. From that point, it's pretty repetitive and is just about a constant back and forth between Monika being genuinely happy about things only to start being sad when she remembers it's all a game. And you're basically trying to get her to see that even if the game ends and the game isn't real, her feelings are real and she can take good memories back to the original game and find happiness this way. And by the time the game does end both literally and figuratively, you've helped her accomplish this.
I personally thought her route didn't build off anything established about her in the original game, and if anything, took a less nuanced lens towards it. I get that it's optimistic and it wants to be but the other routes achieved this while adding additional layers to the characters and building off what was established about them in the original game. This might be a side effect of the original game allowing you to explore Monika's character so deeply in Act 3, but still, I thought they could have done more. The playthrough goes on way too long for what it's doing, it shouldn't have been any longer than Yuri's, the Act 1 stuff not withstanding.
Additionally, the issue I had with a CG in Natsuki's route broke the credits for me as the game wants to load it up here, so I didn't see the credits, and the game reset completely and wiped my save file as a result. I don't know if the save wipe is meant to happen but yeah I can't even go back to watch the credits now.
But here's Monika's CG.
isn't immune mind you, possibly even 
