Let’s look at important things here –
Mercury - Mercury represents communication skills, marketing, calculative ability, younger siblings, hobbies, skills with hands, intelligence, quick decision making, logical thinking, youthfulness etc.
Moola Nakshatra - It is a nakshatra of research, digging deep and knowing the hidden truth of things and people in life. For more information on Moola Nakshatra, please read this - https://www.astrosaxena.com/Moolnaksh .
Sagittarius - As Moola is part of Sagittarius sign, Sagittarius and things represented by Sagittarius are important here. Sagittarius is the 9th sign of Zodiac Belt, hence it represents many things related with the 9th house of horoscope like Religion, Knowledge, Wisdom, Higher Learnings, Gurus, Teachings from your Father etc. Besides it also represents Coaching, Universities, Professors and Preachers, Higher Wisdom, Written Law of the Land etc. Sagittarius has 2 and half nakshatra which are Mula, Purva-Ashadha and Uttara-Ashadha. Sagittarius' lord is Jupiter.
Ketu - As Ketu rules Moola, Ketu's position and dignity is important to know about the overall functioning of any planet in Moola.
Jupiter - As Jupiter rules Sagittarius, their position and dignity is important to know about the overall functioning of any planet in Moola/Sagittarius.
Houses Involved – Also, we can’t afford to forget the houses involved as a planet can be ruling different houses and sitting in different house for every person as per ascendant and planet’s position. To know overall affect, we have to consider the houses involved as those areas of life are being impacted.
Interpretation of Mercury in Moola Nakshatra – Mercury is in enemy sign here and in nakshatra of Ketu. So dignity-wise, .
Mercury is not in a very good strength but in matters of non-living karakas, Mercury here can still give excellent results. It is because of the similarities in traits of Mercury, Moola and Sagittarius. Mercury is all about being curious to get new information, Moola is all about doing the research at deepest level and Sagittarius is all about knowledge & higher education. So, it looks like the best combination possible for them as they all help in gaining of knowledge. So, end result is someone who can be hungry to get more and more knowledge and information. They can do well in any research oriented education. They can also be seen as professors, teachers and counselors. One possibility is being a preacher or someone who talks about different philosophies, religion and literature. Needless to say that door of occultist is always open for them due to Mercury and Mola both representing occult.
At the same time, if Mercury is a relationship oriented planet then it can be the most troublesome position as Moola's deity is Goddess Nritti, the Goddess of Destruction. So, in their relationship matters not only they will go through major destruction or transformation in relationship but their partner may also have a difficult past relationship before meeting them. They can even become relationship counselor to others because of their dealing with so many transformative events in relationship.
Role of Ketu – Ketu placement is important to guide Mercury in right direction, being Moola's lord.
Role of Jupiter - Also, Jupiter placement is important to guide Mercury in right direction, being Sagittarius lord.
Other Factors – And of course, we need to consider conjunction, aspects, house lordship, house position and dashas etc to know the exact results of Mercury in Moola. So, we always need to take a holistic view of chart rather than just 1 planetary position.
Conclusion – So, this is how I see Mercury in Moola can work in a chart.