When alkyl substituted acetylene undergoes addition with hydrogen in presence of Lindlar's catalyst, the alkene formed is 

Lindlar's catalyst is nothing but finely powdered Pd in the presence of barium sulphate.

Here, on reaction of alkyl substituted acetylene with hydrogen in presence of Lindlar's catalyst gives alkene. 

Considering dimethyl substituted acetylene as alkyl substituted acetylene,

${\mathrm{H}}_3\mathrm{C}-\mathrm{C}\equiv \mathrm{C}-{\mathrm{CH}}_3+{\mathrm{H}}_2+\mathrm{Pd}/{\mathrm{BaSO}}_4\to {\mathrm{H}}_3\mathrm{C}-\mathrm{HC}=\mathrm{CH}-{\mathrm{CH}}_3$

The methyl groups are attached on the same sides of the alkene carbon chain hence, showing a cis-isomerism.