You are right that even in La Liga Sevilla did not appear as serious as in the Europa League competition. No one would be surprised how strong Sevilla's DNA is in the Europa League. Seeing from the beginning Sevilla appeared in the Europa League until they reached their peak in the Final 50% of them locked in a win even though the opponent was Roma. They have won 7 trophies which makes the next season quite interesting as many Premier League challengers like Liverpool and Brighton can show resistance.
I think this is one of the intelligence of Sevilla coach, they know this season is not a good season for their team in La Liga, they are only able to be in 11th position in the standings. Making the focus on UEL is the right decision. Even they are struggling in La Liga, they were able to maintain their position in UEL until they succeeded in becoming champions. AS Roma lost in the UEL final, but I think this is a sign that Mourinho has been able to improve AS Roma slowly.
If Sevilla is not serious in La Liga, then it is very likely that Sevilla will be relegated, because at first Sevilla was able to recover or get up too after making many attempts in La Liga matches. With great efforts that continue to be made, Sevilla can finally avoid the relegation zone in La Liga and what's more, Sevilla can win the Europa League title this season by getting rid of other big teams like Manchester United and also Juventus and not just AS Roma.