Do you mean it leaks when it's on the side stand or when it's fallen over? If it's the latter then, yes it's the fuel in the float chamber coming out of the overflow pipe. It shouldn't be a great deal if you pick it up fairly quickly.
If it's leaking when it's parked on the side stand then it's the needle valve in the carb float chamber sticking open and , no it shouldn't do that. The needle valve is supposed to maintain a constant level of fuel in the float chamber and is designed to shut itself off as the fuel level there increases.
A bit of dirt in the valve or if it's a bit gummed up from the fuel evaporating through standing too long and it can stick open. Either way you need to sort it out as a. leaking fuel costs money. b. fuel coming out of the overflow pipe can end up on your back wheel, and c. if the overflow pipe gets blocked / is overwhelmed the fuel can end up inside your engine with potentially expensive results.
Sometimes just tapping the side of the floatchamber with something like a screwdriver handle can sort out a slightly sticky valve. If not you'll have to take it to bits and clean it. Guess what I've got to do on my bike in the next few days?
I've got an XR600 and if your 250 is anything like it for hot starting / flooded starting (ie a pig!) you need to learn "the system". Once the bike is back upright - 1. turn the fuel off. 2. pull in the decompressor, fully open the throttle and kick the engine over at least five times rapidly. 3. close throttle, release decompressor, turn fuel back on and ease engine just over tdc. 4. kick to start with throttle completely shut. If it doesn't go, ease it over tdc and try a second time. 5. If it doesn't start on the second kick, go back to 1. and start the whole procedure again.
For a cold start you need to add turning on the choke to 4. and turning off the choke to 1. but going round and round that cycle has never failed to eventually start my bike. It can be a bit of a pain though when you're up against a time limit such as stalling it at traffic lights.