Well, first of all, there haven't been a ton of 30 goals/season Ligue 1 strikers, and they'd usually have similar or better stats in the other leagues they'd go to. England is not a much harder league, it's very easy to transition to that league, or the Spanish league for players of that caliber that played in France before. The difficulty of your league is a complete myth sustained only by the sum of money spent on players as some sort of indicator of difficulty. You're a rich league, not a tough one.
I really don't think it would be any harder for Mbappe, because the only clubs that could afford him would most likely be City, he might even score more in England than France, it really wouldn't surprise me. Mbappe just turned 20, most players don't move until they're 21-22. There's no rush, and it's a bigger accomplishment for him to win the CL with his hometown's club than it would be to add another one to Madrid or Barcelona which is where he's most likely to go next. Everyone knows he's not going to spend his whole career in France, that's not even a remote possibility, but transitioning to English football is not this big challenge some of you'd like to think for players like Mbappe. He'll be in Spain within the next 2-3 years at the most, I just like that he didn't want to leave without trying to make history with his hometown first; that's more important to some in France than feeling a need to prove some doubters wrong because he hasn't done in X or Y league, nevermind he's shown in the CL and world cup already that he can be effective in any context.