Isn't it always like that?
Real Madrid are always the favorites to win a derby regardless of the competition, unless 1/4 of the starting lineup is out due to injury. I agree that Real Madrid are favourites, but Atletico Madrid are still dangerous despite trailing 1-0 in the first leg.
Real Madrid wasted a chance to score a second goal in very last minute of the first leg and it gives Atletico Madrid more hope of making a comeback in a second leg. Simeone let his players focusing on defense in like last 15 to 20 minutes of that match, perhaps to secure a minimal defeat as their good preparation for a second leg.
Trailing that match with only 0 - 1 defeat, gives Atletico Madrid good opportunity to at least equalize an aggregated result in the second leg.
Atletico Madrid's chances of qualifying for the next round are still very big, so of course the derby match is still difficult to predict. I believe Real Madrid may have a better chance of qualifying, but they still have to take the lead from the first half. It's just like Atletico Madrid's mentality will be damaged, but if Atletico Madrid takes the lead first then their strength will increase.
Yes, it's good enough, and firstly if they can equalize an aggregated result, many things might come after that. Real Madrid are very strong and experienced, and sometimes they can bounce back strongly after getting two or three conceded goals. Atletico Madrid need to score goals, win the match but also have to pay attention on defense to avoid conceded goals from Real Madrid.