Why is this molecule considered a cis isomer of 2-bromobut-2-ene?
With the same reasoning as provided here, I recommend that you name this compound (2 E)-2-bromobut-2-ene.

Using (cis/trans) may be useful around an isolated double bond with only two substitutents. While in the example presented by you the two methyl groups are on the same side of the double bond, they are not the only ones to be considered here. Triple and quadruple substituted dienes should be named in accordance to the CIP rules only.

Edit by 02-Jan-2019:
Agreeing with a comment by @Loong, the previously suggested name, "(E)-bromo-2-butene", was replaced by "(2 E)-2-bromobut-2-ene".  This edit removes simultaneously the ambiguity about the position of the bromine, as well as the double bond.  The interested is referred to the illustrated examples of rule R-7.1.2 of the corresponding Blue Book here.