Sounds weird, ok.
Mandalorian get bonus if use blaster rifle because the background…
Mandalorian get a good heavy armor to start…
But the perk system related with blaster rifle only give bonus if you to use light armor. Not heavy.
To make this fair, the player choosing mandalorian background with the intention to focus with blaster rifle directly can get a way to exchange previous heavy armor for a version of light armor of the same armor. The new characters can get a item, giving a window with a small option, to choose light or heavy armor.
Ok, mandalorian have a good background about becoming the best warriors (this give the vague idea of melee focused warriors) but using blasters is a common place too.