Can a slim girl beat a muscular girl in a boxing match or kickboxing competition even if the other girl has bulging muscles?

Maybe. While skill does play a large part in such competitions, strength and stamina have their part to play as well. If both have been training for a long time it is likely to go to the bigger one. If the slim girl has more training it will be more of a toss up. If the bigger girl has no training, the slim one may be able to win on skill.