Indeed, Leipzig is more promising than Frankfurt in terms of performance and this can be a pretty good reference for those who want to place bets on the DFB Cup final. Maybe Frankfurt was lucky enough to beat Freiburg in the last weekend of the bundesliga yesterday, but that still hasn't proven its capacity to beat Leipzig.
Especially if we look at the odds offered by the bookie, it's quite clear that Leipzig is favored to win with odds @1.73 and for Frankfurt to get odds @4.30, it's clear that the difference is very big, so I don't think there's anything wrong with betting on a team that will clearly win the DFB Cups this season.
RB Leipzig definitely is the favorite in the final of the DFB cup against Eintracht Frankfurt. That being said, i don't think that the chances of Frankfurt to win the cup are that small to be honest. First of all it is a final and in 90 minutes everything can happen. An early goal for Frankfurt would change the whole game. Also Frankfurt has proven multiple times now that they are able to beat better team in cups. They did this last year against FC Barcelona in the Europa League and they have also won the DFB cup against Bayern Munich a few years ago.
It is also the last game for Frankfurt with their coach, so this could give another boost for the players i can imagine.