1) Is the boss readable?
Mostly yes, but I think the attack animation should be different than the one used for the player, it feels too short/fast for the boss.
2) Is the boss fair?
I think he is, maybe add a more clear telegraph for the action he's gonna do after the ultimate move.
3) Is the boss fun?
Yes, but the first phase is too easy. Maybe if this enemy later on can be like a mini-boss or regular enemy it would make sense, but the ultimate would be hard to implement I guess. Also, the telegraph for the ultimate move could be reduced.
Another interesting idea is for him to be able to chain those ground slams:
First he does one on the center with the same long telegraph, then one on each side of the screen but with much less telegraph time, could be interesting.
4) Is the boss memorable?
Design-wise it gives me the impression of a mini-boss more than a boss. Moveset-wise I think it's good, but could be improved even more!
More general feedback:
-Not sure if it's a design choice, but I think you should be able to jump even if an attack is in progress. As it is now, if you start an attack while grounded, you can't jump until it's finished. Since you can attack in the air, I think it makes sense to be able to jump while the grounded attack is happening.
-Also, not sure if it's intended, but you can kinda cancel your grounded attack by dashing. I haven't used this in the fight itself but it could be a nice feature if you want. If not, you could also prevent the dash while the attack is active