Disclaimer: This answer is from a Vaishnava perspective and is not intended to diminish or invalidate other traditions or viewpoints.
Om Namo Bhagavate Vasudevaya!
Is the goddess Lakshmi powerful?
She is ParaBrahman and Bhagavati Herself, so I’ll let the scriptures describe Her glory:
The Lakshmi Tantra states:
mahālakṣmīḥ samākhyātā sāhaṃ sarvāṅgasundarī mahāśrīḥ sā mahālakṣmīścaṇḍā caṇḍī ca caṇḍikā bhadrakālī tathā bhadrā kālī durgā maheśvarī triguṇā bhagavatpatnī tathā bhagavatī parā
Translation:
“She is called Mahālakṣmī, the all-beautiful one with every limb perfectly formed. She is Mahāśrī, Mahālakṣmī, and also Caṇḍā, Caṇḍī, and Caṇḍikā. She is Bhadrakālī, Bhadrā, Kālī, Durgā, and Maheśvarī. She is the three guṇas themselves, the consort of Bhagavān, and the supreme Bhagavatī.” - Lakshmi Tantra 4.39-40
The Sri Suktam states:
dhanam agnir dhanaṃ vāyur dhanaṃ sūryo dhanaṃ vasuḥ dhanam indro bṛhaspatir varuṇaṃ dhanam aśnute
Translation:
“O Mother, You are the Power behind Agni; You are the Power behind Vāyu; You are the Power behind Sūrya; and You are the Power behind the Vasus. You are the Power behind Indra, Bṛhaspati, and Varuṇa; You are the All-Pervading Essence underlying everything.” - Sri Suktam 20.1-2
The Devi Suktam states:
a̱hame̱va vāta̎iva̱ pravā̎myā̱rabha̍māṇā̱ bhuva̍nāni̱ viśvā̎
pa̱ro di̱vā pa̱rae̱nā pṛ̍thi̱vyai tāva̍tī mahi̱nā samba̍bhūvaTranslation:
“I only breathe forth, like the wind, while holding together all living creatures. I am so vast in greatness, that I am beyond heaven and this earth.” - Rig Veda 10.125.8
The Srimad Bhagavatam says:
yuvāṁ tu viśvasya vibhū jagataḥ kāraṇaṁ param iyaṁ hi prakṛtiḥ sūkṣmā māyā-śaktir duratyayā
Translation:
“My Lord Viṣṇu and mother Lakṣmī, goddess of fortune, you are the proprietors of the entire creation. Indeed, you are the cause of the creation. Mother Lakṣmī is extremely difficult to understand because she is so powerful that the jurisdiction of her power is difficult to overcome. Mother Lakṣmī is represented in the material world as the external energy, but actually she is always the internal energy of the Lord.” - Srimad Bhagavatam 6.19.11
The Vishnu Purana states:
nityaiva sā jagan-mātā viṣṇoḥ śrīr anapāyinī yathā sarva-gato viṣṇus tathaiveyaṁ dvijottamāḥ
Translation:
“She is the eternal mother of the universe, the goddess of fortune of Lord Viṣṇu, and she is never separated from Him. In the same way that Lord Viṣṇu is present everywhere, so is she, O best of brāhmaṇas.” - Vishnu Purana 1.8.15
The Devi Bhagavatam states:
“She (i.e., Lakshmi) is of the nature of Śuddha Sattva (Higher than Sattva Guṇa) and is Kṛṣṇa’s Presiding Deity of all wealth and prosperity. This very beautiful Lakṣmi Devī is the complete master of the senses; She is of a very peaceful temper, of good mood and all-auspicious. She is free from greed, delusion, lust, anger, vanity and egoism. She is devoted to Her husband and to Her Bhaktas; Her words are very sweet and She is very dear to Her husband, indeed, the Life and Soul of Him. This Devī is residing in all the grains and vegetables and so She is the Source of Life of all the beings. She is residing in Vaikunṭha as Mahā Laksmī, chaste and always in the service of Her husband. She is the Heavenly Lakṣmī, residing in the Heavens and the royal Lakṣmī in palaces and the Griha Lakṣmī in the several families of several householders. O Nārada! All the lovely beauty that you see in all the living beings and all the things, it is She; She is the glory and fame of those that have done good and pious works and it is She that is the prowess of the powerful kings. She is the trade of merchants, the mercy of the saints, engaged in doing good to others and the seed of dissensions in those sinful and vicious persons as approved of in the Vedas. She is worshipped by all, reverenced by all.” - Devi Bhagavatam 9.1
Why are there so many stories that belittle her?
This is Her leela (i.e., divine play).
These stories may sometimes apparently belittle Her to us, but there is a deeper reasoning.
Hare Krishna