Write a Python function `is_sorted(lst)` that checks whether a given list of numbers is sorted in ascending order. The function should return `True` if the list is sorted and `False` otherwise.
#### Example Usage:
```python [main.nopy]
print(is_sorted([1, 2, 3, 4, 5])) # Output: True
print(is_sorted([5, 3, 2, 1])) # Output: False
print(is_sorted([1, 1, 2, 2])) # Output: True
```
#### Constraints:
- The input list will only contain numeric values.
- An empty list or a list with a single element is considered sorted.