I don't think Mbappe will ever be on Barcelona's radar. Currently, the Blaugrana almost have a mature squad with unstable financial conditions. I think the arrival of Gundogan is enough to complete the Barca squad to return to control of La Liga. Osimhen could be the right option if Xavi wants to add strength to his attack line, but the price is too high, of course the team from Catalan cannot afford it.
Barcelona can't put expensive players on their radar for now. Because the FFP rules are a big hindrance for them to make bigger signings. Otherwise they were targeting to bring back Messi to the team. They were only going to need to offer a contract to him without a transfer fee as he was free agent. But they couldn't even perform that as a result.
Let's say these rules allowed them to make big signings now. Still I also doubt whether they have enough funds to afford Mbappe's demands and PSG's asking price both. His asking price must be around 200 million euros and I can't even imagine the whole package with his contract conditions included.