I agree that Porto is not a team that can be underestimated, because it can beat Juventus, that's an extraordinary achievement in my opinion.
But compared to other teams that qualified for the quarter finals, it must be admitted that Porto is indeed the weakest. So maybe that's why
Chelsea are quite happy in the quarter finals to face Porto. But that doesn't mean Chelsea think Porto is a weak and easy team to deal with.
Because all the teams that made it to the quarter finals are teams that have good quality and are very strong.
Porto have never been a weak team in Champions League after the season they won the cup with Mourinho, Deco, and some great players. They are one of very familiar team in Champions Leauge and usually go through Group stage round. They play very closed matches with Juventus and with this style they will bring troubles to Chelsea.
Chelsea with Tuchel have improved a lot on results and their tactics. They less naively play like the time with Lampard and it seems their defense system has been better. Porto will be a test for them as Porto will take advantage of small personal or systematic faults from Chelsea.
Porto has the potential to beat a team like Chelsea. They have been able to reach this round by defeating a strong opponent like Juventus.
They can but not easy and it can be another close result with home away goals.
However, we cannot judge Porto as the favorite team to win in this match because they defeated Juventus. Remember that Porto didn't beat Juventus easily, it was a pretty tight match. Also, the style of play of Chelsea isn't the same as Juventus, so it is potentially surprised the players of Porto. The match is still difficult to predict, mate!
They will not only have to beat Chelsea but many other greater and stronger teams are waiting: Manchester City, PSG and Bayern Munich. It is possible but small probability as to win the Champions League, many conditions need to be met: strong, luck, and excellent moments from players to score decisive goals.