Adding to what other's have said, I've made a few observations concerning For Glory, and I agree that it's not the best way to get better.

A lot of the people on for Glory play just like slightly smarter level 9 CPUs. I mean, all the power in the world to them, they have great timing/reflexes, but playing them won't get you any better at reads.

Additionally, good For Glory players tend to try and run up their win% on people who are new to the game by playing "formulaic." They know how new players are going to move and will keep doing the same thing to get wins.

Now, a good thing about playing people like this is that you'll figure out the "formulas," and it's quite satisfying to beat people who are clearly a bit better than you at movement and timing. However, you're only going to get better at beating that one repeated sequence, then they will probably leave the room. This is what /u/SubtleTypos was getting at by saying "at some point, you'll actually start moving backwards as you run into more roll spammers." There's just a threshold where it's too similar to playing a level 9.5 CPU.

Another thing that's kind of me being PJSalt y is that the lack of platforms is going to lead to a lot more "spam." Play options are simply way cut down playing flat stages, and people will do what gets them wins against new players that can't adjust. You'll see way more projectile+dodge roll spam because of the lack of platforms. (Seriously, Toon Link players are sadists.) Even if you are winning you won't get better playing against this for the same reasoning that it becomes "formulaic" like a CPU.

For Glory isn't totally worthless, though. The key is to find a good partner and stay in the room with them. There are some other people like you out there looking for a good fight.