EL Classico will only be exciting when Barcelona and Real Madrid have almost equal player skill. Mbappe's Real Madrid will undoubtedly lead Real Madrid ahead of Barcelona.
Barcelona used to have enough money which allowed them to buy players on par with Real Madrid but they don't have that opportunity at the moment as they are suffering from financial problems. If Mbappe is signed by Real Madrid next season, Barcelona will sign a player considering Mbappe's power, speed and skill. EL Classico will be exciting when the two teams are as aggressive as ever with neither team conceding a single point and talking to their opponents eye to eye. Only if they come back will EL Classico be as exciting as ever.
It will only be a friendly match, so don't give this match the same importance as the official matches. Mbappe is able to strengthen the attack of Real Madrid, but I think that now in football the value of one player for the team is a little overestimated, the team game will decide a lot. Nothing is clear with Mbappe yet, but it looks like PSG have definitely decided to sell him, since he does not train with the team and does not play friendly matches.