Raksasha marriage was practiced among Kshtriyas, it is kind of immoral. Lets have a look at what Raksasha marriage is as mentioned in Manusmriti:
3.33. The forcible abduction of a maiden from her home, while she cries out and weeps, after (her kinsmen) have been slain or wounded and (their houses) broken open, is called the Rakshasa rite.
As you can see here, the women is all crying and weeping but if Subhadra abduction is immoral or not is not known because in Mahabahrata it is nowhere mentioned that Subhadra was weeping or resisting or that it happened against her will-
Meanwhile Subhadra, having paid her homage unto that prince of hills, Raivataka and having worshipped the deities and made the Brahmanas utter benedictions upon her, and having also walked round the hill, was coming towards Dwaravati. The son of Kunti, afflicted with the shafts of the god of desire, suddenly rushed towards that Yadava girl of faultless features and forcibly took her into his car. Having seized that girl of sweet smiles, that tiger among men proceeded in his car of gold towards his own city (Indraprastha). Meanwhile, the armed attendants of Subhadra, beholding her thus seized and taken away, all ran, crying towards the city of Dwaraka.
Infact Vyasa here addresses her as 'girl of sweet smiles' While someone is weeping this kind of addressing is not made in Mahakavya. For example in Ramayana a hopeless,beautiful girl who was weeping after being raped by Ravana is referred as 'crushed lotus'
Also here is a reference from Srimad Bhagvatam which says she also fell for him-
(6) With his eyes wide open of happiness, he saw the wonderful girl there who enchanted heroes. Smitten he fixed his mind on her. (7) Seeing him who stole each woman's heart, she desired him also. She fixed her heart and eyes upon him and full of desire bashfully smiling cast sidelong glances. (8) Thinking of nothing but her, Arjuna awaited the right opportunity. With his heart trembling of the strongest desire, he could find no peace. (9) When she during an important religious festival rode away from the fortress in a chariot, the mighty warrior seized the opportunity to abduct the girl who had stolen his heart. That occurred with the consent of her parents and Kṛṣṇa.
Srimad Bhagvatam 10.86.(1-9)