First of all, there is a lack of effects. There are so many zoom-ins and zoom-outs, transitions every now and then, and that's about it, from what I've seen. I remember there was a very old version of Toxic (probably deleted now) that had many more filters and effects that made it really feel like the video was toxic.

Second: Zooming in/out, and Ken Burns/panning effects. Like mentioned above, there were many zoomins and zoomouts, as well as the Ken Burns/panning effects. Keep in mind that if you do it too many times, the viewers may get dizzy or sick. I do like the screen going sideways, but you might want to limit the redundancy, since so many transitions are just built on this. Try adding more effects, movement with the mice, and just more overall action. Play and experiment with filters, see how they can be used to make the overall mood and theme more attached to the music.
And, yes, I know that the camera mostly follows the mouse in action so it wouldn't look like a "Let's Play" video. But excessively zooming in is hard on the eyes, especially when the character is literally flying across the screen.

Third: Lack of movement. Again, mentioned above. There is an extreme lack of movement in every time the mice appear on the screen. Most of the time, they literally just use an action, duck, or run a bit before the next bit comes along, which is even less emphasized because of the constant following of the camera. If you could do something else, such as walljumping (as you did in a few), cornerjumping, and turn-around'ing (though not excessively) it would make it a bit easier to watch. Of course, this doesn't need to happen during the lyrics; most songs that I've seen have little instrumental parts of it, so maybe you can fit things in there. Also don't forget that there are consumables, such as the fireworks and the "farts".

Fourth: Too many cutscenes/jumpcuts. These often make viewers dizzy as well. This might be one of the reasons of why there has to be so much zooming in and etc. If you could make each segment a bit longer (and yes, I know, the person acting out on camera has to copy/paste more lines or do it faster, but hey it helps) per mouse, it would be more pleasing to the eyes.

Fifth: The outfits are kind of random. I mean, I know that we all have a budget of cheese or fraises and can only afford so much. But most of the outfits weren't exactly appropriate to the theme of the song. Maybe in Toxic, ALL of the mice could have been the bright green. Also, the main character, presumably the lead singer, should have been the same outfit. I know that all mice share a part of the whole song which usually has only 1-2 singers but it was completely random, all the guys dressed up as guys and girls dressed up like girls. Now, I'm not saying it's necessary, but it was slightly confusing to see random mice pop up out of nowhere and having lyrics over their heads. I mean, it doesn't have to be a solid story with a main character and all but many outfits were unrelated to the song; e.x. Hey Soul Sister, some of the "sister"(s) looked NOTHING alike. They didn't have to be twins, but they didn't match up too well so naturally I was confused at first.

Sixth: Where did the punctuation go? Sometimes punctuation can put feeling into words. Take these lines from Dollhouse as an example:
hey girl...
open the walls...
play with your...
d o l l s...
If you don't recognize the "creepy" mood I tried to insert into the lyrics, you should at least notice the differences. I put ellipses to symbolize the petrifying voice that Melanie Martinez embedded into the music. Also notice the spacing between the letters in the word "dolls", which symbolizes both the end of the phrase, music-wise, and the horrifying themes that came with the softness but aggressive word.
Let's try another:
Baby, can't you see,
I'm CALLING...
A guy like you should wear a WARNING;
It's DANGEROUS,
I'm
f a l l i n g...
For this example, the first thing you notice is the array of capitalization. I emphasized (or tried to emphasize) the mood again, but this time with a different technique. Though Britney Spears doesn't really accent it in the song TOO much, you can still notice how she hoolds out the woords that she neeeds to emphasiiiize, and breathes in at certain points (e.x. callinn...; warninnnn...; a breath right after "dangerous", to lean more to the "falling" part, which is the end of a phrase.)

Seventh: There was only 1 singer at a time.In every single video, there were a few mice who sang the lyrics. There were a few videos I remember that the 2012 Transformods team made for Meli, Tig, or Sydo (I don't remember and I can't find the video x-x) featuring 20+ mice singing the same exact thing. Sure, it's kind of bland when I say it, but when you see it, it's kind of cute. Here is a video I found that's somewhat similar to the ones I remember.

Spoiler

So, in this example. you can see how there are multiple mice that are doing the same actions and have the same lyrics. Though the video is kind of bland, I do think it's kind if cute (sorry for the redundancy of these words).

Eighth: Interference with text. This is only a small thing, but sometimes, the emotes of the mice () are blocking the text. If you could either put emoticons, (e.x. :) :> :] c: :c :[ :< :( :P :p >:c >:( D:< :D :O C: B) 8) (B ://o c//: ^^ ^^' ^^" ^-^ ^_^ :3（。v°） O.O O_O) it would be a bit easier to read the text. Also keep in mind that using too many emoticons might take the professional "vibe" away from the video, but that's just how I see it so do whatever you want. There are also a couple of instances when there are foreground objects or other mice blocking the words so yeah. Try to refrain from that.


Yeah, that's all I got. This is one heck of a wall.

However, overall, nice work on the editing. Please remember that this is purely my opinion so just take it as a grain of salt. I know that it's hard work editing and making all of this so please do know that even though I pointed out most of the flaws that I saw myself, there are many more aspects of these videos that I love (A+ on the /watch command in use, I also love the use of moving maps and you're extremely good at syncing the words with the music).
I also apologize if there are any points that offended you.