There is a technique to topping out a flat featureless boulder gracefully. Climbing.com have a good article on it
Briefly this is:
Step one. When you reach the lip of a boulder, quickly evaluate which
foot to swing up onto the lip (from now on this foot will be known as
the pivot foot). Let gravity work for you by swinging your pivot foot
onto the low side, not the high side, of the boulder. If the lip is
horizontal, pick your stronger — or favored — side to swing up. Do a
half pull-up, and simultaneously raise your pivot foot over the lip,
keeping your knee slightly bent. Try to find a good heel hook, edge,
or foot scum to use as a fulcrum — extra purchase is useful for
floating a mantel. If possible, use your other foot to edge on the
face of the boulder.
Step two. With your arm opposite your pivot foot, reach out laterally,
approximately a forearm’s length, and grab a hold on — or slightly
above — the lip. Palm the rock with your fingers facing slightly
towards your body. Flag with your non-pivot foot to keep from
Step three. Contract your pivot leg so that your hips are even with —
or higher than — the lip. Hold yourself in position with friction from
your pivot foot. Bend your elbows, and use your ab muscles to bring
yourself aggressively close to the rock. If you’re having a hard time
making your foot stick, try draping your pivot-foot leg over the
stone, contacting the rock with the underside of your calf muscle, to
create additional friction.
Step four. Pop your pivot-foot hand and elbow 90 degrees so that your
fingers face your torso. Shift your other hand 45 degrees outwards
(away from your body), and switch your fingers from clasping the rock
to palming it. This positions your shoulders over your wrists, and
improves your overall leverage.
Step five. Rock up, pulling with the thigh and calf muscles of your
pivot leg, keeping tension with your abs. Turn your pivot knee in
slightly, and pull your torso over the lip. Flagging your non-pivot
foot helps as a counter balance, as does smearing (backstepping) on
top of the lip with your non-pivot foot. If done properly, your body
will be almost horizontal to the ground.
Step six. Dig the inside edge of your pivot foot into whatever feature
it’s on. If there are no features, firmly push of your shoe into the
rock to maintain friction. Now lever up onto this foot, twisting your
torso inward. Drive hard with your elbows as you push your shoulders
over the lip with your palms.
Step seven. Extend your elbows and stand up. You’ll likely have to
make minor adjustments in the position of your hands and body to keep
your balance as you go from perching to standing.
on it's web site going into details of this technique.