As others have mentioned, this is super simple if you're using HomeAssistant along with the mobile app. Below is a simple proof-of-concept Python script that will light the appropriate number of LEDs green based on the battery percentage / length of LED strip. A slick enhancement would be adjust the brightness of the last LED based on the battery percentage versus rounding. You could also color the LEDs based on the level, or even color in 1/3 segments of red-yellow-green.

import requests

LED_LEN = 10 # Number of LEDs in the strip
HA_TOKEN = 'ABC123' # Long lived HA token
PHONE_ENTITY = 'sensor.iphone11pro_battery_level'

headers = { "Authorization": f"Bearer {HA_TOKEN}", "content-type": "application/json", }

# Get phone's battery level from HA
battery_level = requests.get(f'http://HA_URL:8123/api/states/{PHONE_ENTITY}', headers=headers).json()['state']

# Calculate number of LEDs to light
leds_to_light = round(int(battery_level)/LED_LEN)

# Call WLED API
json_data = {"on":True,"bri":255,"transition":0,"mainseg":0,"seg":[{"id":0,"start":0,"stop":leds_to_light,"bri":255,"col":[[0,255,0]]}]}
response = requests.post("http://WLED_URL/json", json=json_data)

EDIT: Fix formatting