Quote from RagnarokOathbreaker >>

It it really dumb that this is attunement.

 

 No attunement is mentioned above...and indeed attunement isn't required.

Teleport moment

i love it!

since it casts Dimension door, indeed you can

 This property of the cape can't be used again until the next dawn. <---- Happens at dawn.

very cool. in my campaign it has been stolen by the biggest thieve gang and the players find it in transport to the safe house. Perfect item for the plot

I don't see why this is the same rarity as the Helm of Teleportation.
Can someone explain why?

rarities in 5e are pretty loose in general. however, a big difference would be that the Helm of Teleportation requires attunement, while the Cape of the Mountebank does not. Definitely not exactly even, but that draws them quite a bit closer.

Looks sick

In my next game I will play a barbarian storm herald gnoll whe start level 3 and my dm allow to us start whit 2 common rare item or 1 rare item I decide start whit that imagine I depop and repop behind the ennemy so I have advantage du to surprise (I literaly teleport behind him) and I strike him whit a greathaxe omg I'm gona have fun whit that

TL:DR - This cloak stinks!

Now I have to wonder: this cloak smells of brimstone.  However "slightly" it smells of sulfur (brimstone just being a synonym for sulfur), humans can detect sulfur at 1.5 to as little as 0.01 parts per million (other humanoid (DM's discretion) may or may not be similar).  And though it may or may not (again, DM's discretion) impose disadvantage on stealth checks, with two or more humanoids present, one or more is gonna look around and say, "Was that you?"

In other words, now matter how far into (or past) your rogue rolled a stealth check into the 30s, someone is gonna notice a distinct change in the air from redolence to fetor (SAT words of the day!)

If someone in dnd land suddenly smells sulfur, they're going to think "devil," not "rogue wearing a cape." You might be able to use that to your advantage.