Alrighty then, here is that critique that you requested and have been patiently waiting for.
My critique of constructive criticism is designed to help one improve. But, at the same time, I won't strip you down in negativity. Also, since this is your first comic, don't be disheartened if you wind up with a bad grade. We all have to start somewhere! ^_^
That being said, here's the review you requested:
Sprites: 8/10
A very well-done usage of sprites! I love how they are positioned, and the angles you made with them! There were only two problems I saw:
1. You had the sprites look too large too often. Since 32 bit sprites look super ugly when they're that large, I suggest you use such a technique sparingly to avoid eyesore. Panels 3, 4, and 7 seemed to use that very well, but the others could have been fine without.
2. Mario magically appears on the ground with no indication that he has fallen, other than him saying so. This is something you should avoid in the future. Lack of animation is a big negative factor that separates sprite comics from other web comics. I suggest that you create a sequence of one to several panel in which Mario is seen falling over. This will help in other areas too, which you will soon see.
Background: 4/10
Ouch. You didn't do too well in this area. Lots and lots of white glare, with the actual background only showing twice.
Tip: It is possible to enlargen the background as well. And if you do that, you won't have panels looking the same, but different chunks of the background. This is a good thing and helps the comic not become boring (on the topic of the colored background) and repetitive.
Special Effects: 6/10
You really didn't have any animation special effects to speak of, so I listed the New Super Mario Bros. reference and the Rockin' G-Ma as special effects.
Tip: Earlier, I was talking about Mario magically appearing on his back. Well, this is where special effects come in. Since I assume you're using Paint, you could draw a few lines with an "OOMF!" sound from Mario as he falls. Then, in a seperate panel, he could have said "Help! I've fallen and I can't get up!"
Text: 6/10
Having colored text all the time is not the way to go. Unless used sparingly, and not all the time like in this case, it becomes a real eyesore. It also appeared very crammed, with the font jumping in different sizes throughout the comic. Well, at least it was readable. That's good.
Tip: Try keeping the font the same size for convenience unless you enlarge it for a good reason (like someone shouting).
Grammar/Spelling: 10/10
Perfect! Well, I didn't find any problems with it anyways. Be sure to remember that spelling and grammar is a crucial element in how much one can enjoy your comic. Good job! ^_^
Punctuation: 9/10
You did a very good job on this as well. Just one thing I wanted to point out though.
1. It is unnecessary to have three exclamations(!!!). If used, it should be sparingly, along the usage of two !!'s. Most of the time, two is more than enough, and three just looks like trash. Sometimes, there's too much of a good thing.
Story: 10/10
I love the whole "Story Time with Uncle Luigi" thing you have going on! I hope you keep up with it. One thing I thought was weird (but didn't mark down for), was why it's called "Luigi's Comedy Club" and not something around the line of "Luigi's Story Time" or something like that. The story's great so far, but doesn't really have anything in relation to the title of "Comedy Club." Ah well. To each, his own.
Balloons: 10/10
For what little balloons there were, they were basic, but I couldn't find anything wrong with them.
Panels: 7/10
I think you could've done better with the panels. Much of the text was crammed.
Tip: Every panel should, most of the time, have bite-size sections of text. That makes reading easier, more entertaining, and more convenient as the reader digests a little at a time from panel to panel instead of having it all shoved down their throats in one. There are ways to get around this, but not in this case. For instance, the first panel could have easily split into two, right after "My brother and I" which is a pause that pretty much cues you to start a new panel. A sprite comic is basically a storyboard. Like a storyboard for a movie. The storyboard is filled with bite-sized pieces that are easier to view and make the flow of the comic a whole lot easier.
Other: 10/10
This comic was nice and clean, in a family-friendly sense, which I appreciate. And for humor. . . I place humor under other because, even though a comic is supposed to be funny when you aim for it to be, people have widely different opinions, so I cram it down in here. Anyways, I really loved the fat (water-filled) Mario! And let's not forget about Rockin' G-Ma! Hilarious! :XD
Overall Score: 80/100
Comic Grade: B
You did very well for a beginner! If you follow the tips I gave you, I promise that you will become ever better. Good job. ^_^