In the Champions League, no opponent should be underestimated by any team, especially when it has entered the knockout stages such as in the quarter-finals. This means that every opponent must be seen more seriously by any team if the team wants to be in the next round and win with enough aggregate goals to be able to qualify for the other round. Aston Villa is a fairly serious opponent and I think PSG also sees Aston Villa as a dangerous opponent because in the Champions League all abilities will always be deployed by any team without any fear, especially if the team no longer has a chance to become champions in its own domestic league.
PSG is strong team in French League and is close to winning record 13th French title, right now PSG is number one team on points table of french league. Villa is an average team in EPL with 12 wins this season and is at number 9th position on EPL points table. Odds must be in favour of PSG for there upcoming quarter final with Villa but still they must take the quarter final seriously. In football anything can happen, we have seen many instances where mediocre team was able to beat top teams of a League.
The comparison of why Aston Villa is not a favorite is because PSG can fight Liverpool in the previous match so that people think that the team that defeated Liverpool who is the leader of the English Premier League standings can defeat the derivatives of the AA team underneath and we see that Aston Villa is in 9th position from the side of the English League standings and I think speculation is very reasonable if it calculates it from there.
But yes I agree that Aston Villa can reach the point now is not underestimated in the Champions League, PSG must be careful with Aston Villa they really like to surprise their opponents.