Haaland definitely has the best chance of all active players, but the big thing about Messi and Ronaldo was their insane longevity at the very top level. We'll have to see how long Mbappe and Haaland can maintain their form.
Cristiano scored his first UCL-goal against Roma as a 23-year-old. Haaland & Mbappe have already a massive headstart in their UCL-career as they've scored around 25 goals.
The biggest reason for Cristiano & Messi being top 2 scorers in UCL history is because they were fortune of playing in a ridiculously stacked teams and were title favorites to win UCL every year. Neither Cristiano or Messi would score 100+ goals if they played for a club of BVB's level.
Look how someone like Karim Benzema is 5th highest goalscorer in UCL-history with 71 goals and only 3 goals away from surpassing Raul. And the latter averaged only 5 goals per season in his UCL-career, never scored 8 goals in a single campaign.
Haaland & Mbappe as the main man in their respective crazy stacked team definitely has the capacity to score as many UCL-goals as Messi & Cristiano. The only question mark would be if they can stay healthy, although it's something that's always you've to consider regardless what topic it's about.
They may, but it’s not likely, because of how challenging it is to score consistently and progress to further stage seasons after seasons, for very long period of time.
For example, Haaland only managed to score 3 CL goals this season, despite scoring around 10 goals each in previous 2 seasons. He may surpass the record if he could repeat 10 goals every season for next 12 seasons or so, however he won’t surpass the record if he could only repeat 3 goals every seasons for the next 30 seasons or so.
Consistency and longevity are the keys, being able to play in a great team is another one, and luck also plays a big part.
Haaland only scored 3 UCL-goals this season due to injuries. Messi & Cristiano didn't play in just a "regular" great team, they were surrounded with legendary all-time great teammates such as Neymar, Xavi, Iniesta, Busquets, Alves, Modric, Kroos, Casemiro, Marcelo, Carvajal etc. Both Real Madrid & Barca during 2010s were far & away the most stacked teams in the world and favorites to win UCL every year. Neither Messi & Cristiano would be top scorers in history if they played on Arsenal, BVB etc