Weren't they selling Boba Fett toys before he ever appeared on screen? "Over-hyping" a cool looking character who turns out to be a minor player is nothing new.
Also, if people build a level of anticipation based on a trailer, that's partially on them. Trailers are going to sell a movie in the best possible light and will often not represent the actual product accurately. Similar to the first trailers - "Who are you? Rey: I'm no one" - which immediately set off a firestorm of speculation and theories on who Rey would be. So many expectations and speculation based on a commercial, and then people are upset when the final product doesn't match what they expected, based on their own wild flights of fancy?
Phasma is featured in the trailers because it's a cool visual. Anyone who assumes that means her character is important, doesn't understand how movie trailers work.
I wish that weren't the case, but it generally is. On the flipside, I really appreciate that the Mandalorian kept Baby Yoda under wraps. Instead of flooding us with previews and 5000 toys before the series released, they allowed people to discover and embrace the character the right way. The 5000 toys can come later. It allows us to like the character, whereas had they done it differently, we might have felt like the show was just a toy commercial, and been less receptive to it.