Is it mandatory to have Mantra Diskha or Mantra Daan in Hinduism to get Moksha as per Hindu scriptures?
Is Mantra Diskha is mentioned in Vedas or Darshan?
Darshanas are not the kind of Texts that are supposed to mention about DikshA.
MokshA is the epitome of spiritual realization and all sorts of spiritual advancement start with the Guru DikshA. This is something that is unanimously agreed upon.
So, it is more or less required.
But that does not mean that everyone who're initiated will get MokshA in that very life itself, as that depends on a lot of other factors. But with initiation the process gets started.
In KulArnava Tantram 14.3, Lord Shiva says:
VinA dikshAm na mokshah syAtta duktam shivashAsane |
SA cha na syAd vinAchArya mityAchArya paramparA ||
Without DikshA there's no MokshA- this is the command of Shiva. And without the Teacher, DikshA is not possible; hence in this regard tradition of Teachers (Guru ParamparA) has been laid down.
Also, you'll find in this answer, that Lord Vishnu is teaching Garuda the same thing (verses are from the Garuda PurAna):
na vedŸdhyŸyanŸn-muktiÅ na ÀŸstra pa¢hanŸd-api |
j†ŸnŸd-eva hi kaivalyaÄ nŸnyathŸ vinatŸtmaja ||
One cannot obtain Liberation by simply chanting Vedas nor by self-study of the Scriptures. Liberation comes from the dawning of pure wisdom alone, not otherwise O Garuda.
Garuda PurAna 2.49.87
& how does this wisdom come is answered in the next verse:
tasmŸt j†ŸnenŸtma tattvaÄ vij†eyaÄ Àr guror-mukhŸt |
sukhena mucyate jantuÅ ghora saÄsŸra bandhanŸt || 2.49.101 ||
Therefore one should obtain the True Knowledge of the Atman from a worthy preceptor. From following the teaching of a Guru, one may easily attain liberation from this awesome ocean of rebirth.
Is DikshA mentioned in the Vedas?
Yes, it is. The Vedic Diksha is called the Upanayana, where the Guru initiates the disciple into the GAyatri Mantra.
In case of Agamic DikshAs, Mantras are many, but in Vedic initiation, Mantra is only one.
The Atharva Veda (AV) Suktam 11.5 is the earliest attempt to describe the process of initiation (DikshA) by a teacher (Guru).
This hymn metaphorically describes the Guru as keeping the disciple in his womb for three nights, impart the power to him and make him grow. Thereafter the disciple gets a new birth.
AchArya upanayamAno brahmachArinam (1)
Krinute garbhamantah (2)
Tam rAtristisra udare vibhati (3)
Tam jAtam drashtumabhisamyanti devAhA (4).
The AchArya initiating the BrahmachArin (1), takes him within himself (AchArya) as a mother the child (2); three nights he carries him in his belly (3); to see him (the BrahmachAri) when born (i.e after DikshA), devAs eagerly come (4).
Here, the Teacher keeping the disciple in his belly for three nights is not literal. It means, the disciple staying in Guru's house for three nights.
In the Tantras, it is mentioned as "AdhivAsa". Without this AdhivAsa, the DikshA is considered as not effective.
So, taking Guru DikshA is the very first step towards to the ultimate step (if you consider MokshA as the ultimate goal). Every Guru, every scripture will say the same.