Not all the time, I have been in mish mash groups that have been dominant against pretty much any premade I have come up against. So it does happen, and it does help if you have an Ops Leader who has communication skills, as this ultimately is where a win for a mish mash group will come from, if against a well drilled premade.
It would be nice if they had an in game communications server, that everyone can get into. This would then possibly lead to many of these premades being put to bed in the long run as communication is to me the key to any win, whether against premade or a mish mash group.
I guess comms is useful, but most comms can be done over reg chat just announcing stuff inc and numbers etc.... the biggest thing premades have over solo que ppl is group composition. 2 or 3 heals and rest dps/tank vs a pug grp that has maybe 1 healer in a blue moon. Of course gear, comms, etc make a diff...just feel the grp make up is the most impt and least contrtollable by the the solo que ppl (how many times have you qued solo and watched a tank call for a healer to protect....and no one answers).