What sort of riding are we talking about? If it was DH maybe you could get away with not having cycle specific shorts (either a short-liner or bibs) as you won't be in the saddle pedaling too much.
If it's a more pedal orientated of the sport (XC), I'd lean towards investing in a pair of cycle specific shorts with a pad in - especially if you plan on anything endurance based. They will give more to your experience/health/comfort than any other component of clothing (except a helmet, but I guess that's a given).

Jeans... On dry trails, maybe... But they're going to get soaking wet and stick to you a lot after anything wet/sweaty.

Although, at the end of the day, ride with what kit you want - nobody is going to stop you riding and the most important thing is actually getting out and riding as far as I'm concerned.