Don’t assume just because someone else is thoroughly convinced of something, that means they are right.
A better speaker can make a ho-hum amp into something you love to play.
Fender tends to use crappy speakers in their amps, especially the entry level, or cheaper amps. Like the Mustang, or the Blues Jr. But even a Fender Twin Reverb, or Hot Rod Deluxe can sound tremendously better with a better speaker. I have never upgraded a Fender stock speaker and not been pleased.
That all said, I’m not sure of your choice of speaker. The Swamp Thing is a real bassy speaker. It actually is recommended for jazz due to it’s lack of high frequencies. They have some nice sounding speakers in the Legend series. My favorite is the Texas Heat, or the Wizard, or the Tonker.
Emminence has a really good website, with sound samples of all the speakers, with different styles of music.
As far as this business of using a neutral speaker on a modeling amp? Yeah, ok, but it’s still a guitar amp. The supposed need for a neutral speaker is, greatly exaggerated. Besides, the stock speaker is crap, it’s from Fender.
Lastly, why burden the Monoprice with Fender junk. That amp would also benefit from a better speaker. Possibly even more than the Mustang.
If you do get a new Eminence speaker, give it some time to break in. They sound harsh initially. But after 20 hours or so they mellow out and get really sweet.
I’ve used the Lil’ Texas speaker in my Fender Twin. It sounds the same as a Texas Heat, but uses the lightweight neo-magnet, and saves some weight. 4 lbs vs 8 lbs.
Use your ears to judge how something sounds. For example
https://m.youtube.com/watch?v=9-bIy0V2M14