The words ‘aerodynamic’ and ‘mountain biking’ are not frequent bedfellows. You do not need to try and be aero on your mountain bike. Therefore, there is no real advantage to wearing lycra – unless you really want to, of course.
The ‘uniform’ of the average mountain biker includes baggy shorts, a loose top, full finger gloves, socks pulled up and a helmet.
If you decide to ride downhill, you may want to get some down hill kit. Be careful when purchasing, as it is easy to confuse downhill kit with pyjamas.