Genetics are important – they contain the blueprint to make your muscle fibres. However those muscles need food to grow so nutrition and exercise is important too. Yes some people carry genes that make them more muscular.
Genetics do play a part alright (as Kieran mentioned) but you can still make the best of what you have and become the best that you can be.
Whether you’re able to gain muscle at an above or below average rate is something that cannot be changed. Unless your goal is to become a world-class bodybuilder (something that is largely dependent upon genetics), absolutely anyone can build a muscular, impressive body as long they are willing to put in the time and energy!
For some people it may simply take a shorter or longer time than others. It’s as simple as that.