Question on Dhammapada Verse 30

During "Vedic Brahmanism", Lord Indra is superior of all gods. Does this verse means that Buddha accepted the existence of Gods?

Sure, but the Buddhist notion of the gods wouldn't be the same as others theistic religions. In Buddhism, the gods/devas, though being very advanced and highly meritorious beings, are still sentient beings not yet able to escape the chain of Samsara, hence still have "work to do".

Also the verse mentions that because of appamada, Indra become the overlord. Does it mean Lord Indra has the insight of Nibbana ? Appreciate your answer.

Dhp 30's context is about worldly merit, the kind of which will get one to be reborn into higher realms. To gain insight of Nibbana, it'd require a much more dedicated, rigorous, and advanced training in Sila/Samadhi/Panna, the Four Noble Truth, and the Noble Eightfold Path. Matter of fact, while Sakka's position as the Lord of the Thirty-three Gods is quite powerful, his realm compared to the overall schematics of all the realms of existence is still relatively un-impressive (it still belongs to the Sensuous World/Kama-Loka). Refer to the Thirty-one Planes of Existence for further details.

There is no evidence the Buddha himself spoke any sutta, apart from suttas that can be verified by meditation to accurately diagnose & end suffering. Therefore, we cannot know if the Buddha accepted the existence of Gods or what the Gods exactly were or meant to the Buddha. For example, in the suttas, Iti 106 says the Buddha said parents are the gods of their children; therefore gods in this context is obviously not supernatural.

It appears "Maghava" refers to "Sakka" in the Pali suttas, where eventually Sakka becomes a disciple of the Buddha. The following suttas are mere examples:

1. MN 37 - Sakka inquiring about Buddha's Dhamma then being warned by Buddha's chief disciple for being heedless in respect to sensual pleasures. Here, Sakka lives in a palace with many female nymphs.

2. SN 11.4, where Sakka is called "Maghavā" (bhayā nu maghavā sakka) when questioned by an asura about how to respond to & manage ordinary people.

3. DN 16 where Sakka expresses Noble or enlightened understanding to the Buddha's passing away: "Transient are all compounded things, Subject to arise and vanish; Having come into existence they pass away; Good is the peace when they forever cease.".

While each of the above may simply be religious propaganda written in religious scriptures (to propagate the Buddha is superior to Indra, Brahma & other Vedic & Hindu gods), each of the above shows Sakka is not eternal, not omniscient nor omnipotent. In fact, MN 37 & SN 11.4 give the impression Sakka is the leader of the rich & powerful, such as an Emperor of Rome, Genghis Khan, Suleiman the Magnificent or Lord Rothschild.

As for "heedfulness", this may not include the higher teachings of Buddhism such as not-self (anatta) that lead to Nibbana. For example, the word "heedfulness" or "carefulness" is found many times in the Jewish & Christian Bible. SN 11.4 shows how Sakka won the debate among the different gods by having heedfulness in relation to being patient. This does not show he was close to Nibbana. It only shows he had morality & was thus only heedful in respect to maintaining his patience & virtue.

Matali

‘When face to face with Vepacitti, is it, Maghava, from fear or weakness, that you endure him so patiently, listening to his harsh words?’

Sakka:

‘It is neither through fear nor weakness that I am patient with Vepacitti. How can a wise person like me engage in combat with a fool?’

Of goals that culminate in one’s own good None is found better than patience.

One who repays an angry man with anger Thereby makes things worse for himself. Not repaying an angry man with anger, One wins a battle hard to win.

‘He practises for the welfare of both, His own and the other’s, When, knowing that his foe is angry, He mindfully maintains his peace.

‘He achieves the cure of both— His own and the other’s.’

The verse is simply praising appamada (heedfulness) and criticizing pamada (heedlessness).

The use of the character Indra here is just for the listener who believes in Indra as the king of the gods.

The Pali suttas do mention gods (devas, brahmas) but none of them are supreme, eternal or immortal. They are also not omniscient, omnipotent and omnipresent.

Whether you believe in gods or not, it doesn't matter in walking the path of the Noble Eightfold Path and understanding the Four Noble Truths.

Dhammapada 30 just provides an example of what is taught in the Ittha Sutta:

Long life, beauty, status, honor,
heaven, high birth:
To those who delight
in aspiring for these things
in great measure, continuously,
The wise praise heedfulness
in making merit.

The wise person, heedful,
acquires a two-fold welfare:
welfare in this life &
welfare in the next.
By breaking through to his welfare
he's called prudent, wise.

Yes, heedfulness in the practice of the Noble Eightfold Path can bring insight, but it's a general term that is also applicable in worldly life to bring worldly rewards too.

Please read this answer to understand what heedfulness is.