I have several questions. Is Dimentio still alive?

"Is Dimentio still alive?"

No. Or at least, you never see him again.

"What is the locked door in Flopside?"

Piccolo.

"Do we ever see Tippi again?"

No. But you can buy Tiptron from Francis in world 3-4.

"How do I defeat the Sammer guys? I always bring lots of HP items, but I always run out buy somewhere past 50. "

See one of the strategy guides.

This is only for the Sammer guy part. You must have alot of good recovery items. I have a strategy that may be time consuming but it is effective once you get to the higher levels. After the battle starts, flip to 3d. The Sammer guy will walk back and forth. Place Boomer near him, flip back and when the Sammer guy comes after you, press 1 to blow the bomb and that way you can beat him a lot easier wihtout that much of a risk. Thats how I did it. Good Luck!
From
Matt c_1

Use bowser's fire breath to kill many of the sammer guys. Ones that don't jump and run towards you, try to hit them with the fire breath but they like to run away so lure them in and catch them off guard. For ones that jump use the pxl slim go invis and wait for them to walk around close to you and than jump on them. This is the easiest way that I have thought of.