You should have about 2 inches of standover on a MTB frame. I ride a Trek WSD 18". I'm 5'6", have no idea what my inseam is, except I have long legs and short torso (traditional build for a WSD bike).

Take the bike for a test ride to determine if the top tube length is really right for you. You should be able to get your weight way forward for steep climbs, and way back for steep descents. If you can't, go for another bike.