Hi there, Nick.
I took a careful listen to your demo here and wanted to take some time to offer my advice. I'll be going at this point by point and try to be as critical but fair as I can.
Firstly, while this is obviously not a professional situation that would require a particular length, a Demo reel should never go beyond 2:00 minutes, unless it is an extended collection of scenes taken from various completed productions. Even shorter than that is preferable. This is so we can get an auditory glimpse of each voice in quick succession, but with just enough time so each voice can resonate with the audience listening. At your current length, each scene lasts a tad too long, especially without other characters in each scene to bounce off of.
Second, I think my overall feeling is that while you are clearly attempting various voices and a certain tone for each scene, your voices aren't nearly as refined as they could be. Think of it like this. I can imagine what your voices here could sound like if you refined them to between 90 and 99.99% (you never really get to 100, we always work to improve our voices), but right now each voice is sitting at 55-60%. You know what the character is, and you know what sort of accent or speech impediment you want to put on them, but your pitch and/or exaggeration is not going nearly far enough. The one voice I felt got to about 85-90% was your Kermit impression, because it sounded different enough from your own voice that it succeeded in standing apart, and suspending my disbelief. So really work at stretching out each of your voices. Practice going squeakier and higher with your head voice, and deeper with your chest voice.
Third, I think in the case of the alien character in the 2nd scene, he's may be too exaggerated, and perhaps too loud. The excessive hyperventilated breathing is a creative choice, but for me, I actually started to feel the rapid breathing myself, which made me uncomfortable. This is a useful tactic when you want a performance to affect the audience in a physical way, you breathe heavily or rapidly to cause a similar reaction. But since this appears to simply be a character quirk you are wanting to emphasize, I suggest stepping back from the mic a big, reducing the length of time you emphasize his breathing, and maybe even make him breathe faster.
Fourth, consider the relationship between your script and the voices you choose. I actually thought the first voice you used was one of the better ones featured here, but it doesn't succeed for me because the character you're trying to play doesn't make sense with that voice. Not that you can't defy expectations, but in a Demo Reel, it pays to work with stereotypes and established tropes rather than against for the sake of digestibility. You want to sell your talent as quickly and efficiently as possible. So every element of your demo has to be in service to that goal, along with being fun to perform.
Fifth, when producing a demo reel, there are common ways that the audio itself is typically EQ'd to smooth things out for the listener. I can't quite tell if any EQ or compression has been done to this track, but if not, that would be something to implement on the next demo. You want to be sure your highest peaks or loudness are brought down in line with your middle frequencies, that way it doesn't spike in the listener's headphones or speakers. Then you can be as loud and intense as you want without negatively affecting the listener's experience.
I hope these critiques do not discourage you in any way, as I always want the people I critique to succeed in what they do. You have a lot of potential here, but it perhaps requires a new approach, and definitely more experimentation.
Sincerely,
FilmmakerJ