There could be several reasons why a text is not looking good on mobile phones.

1. Font size:

When designing a section that includes text and the font size is increased, usually that is done for the Desktop view.
If different values for the Tablet / Mobile view are not defined, then they will inherit the values from the Desktop view.
Here is an example of that: The font-size value was increased to 50px(which is quite big) for the Desktop view, in order to make it more visible.

But if we check it from mobile phones, it's not looking so good, because the same 50px value is inherited from the Desktop view and, on mobile devices we don't have that much space: 

So, in this case, we will simply go and adjust that font size for the Tablet / Mobile devices

Divi allows that by clicking on the responsive editing controls icon and set smaller font size. That will prevent the heading from falling on two lines: 

2. Padding:

The same thing can happen if padding for Desktop is set from the Spacing tab, without adjusting it for the mobile / tablet view too.

It looks good on the Desktop: 

But not so good on Mobile: 

In this case, again, adjust the padding for smaller devices from the Design tab -> Spacing and it will look good: 

Long words:
In some cases, there are some long words set for the heading. Let's take for example a word like ExtraSuperLongWord: 

As you can see, it falls on two lines on Mobile and the font is not so big. In all cases, reducing the font size with the indications from above will work. 

But there is a CSS trick also which can be used to stop the words from breaking on Mobile and to show them in one line. 

You can apply  a normal overflow to that heading by adding this in Advanced > Custom CSS > Main Element :

overflow-wrap: normal;

And the text will stop going on two lines: 

The side effect of this will be that the padding is slightly different in the right side of the text, but the advantage is that if you have lots of pages in your website with lots of headings wrapping, you can target them all from the Custom CSS, by adding this code: 

h1, h2, h3, h4, h5, h6 {
    overflow-wrap: normal;
}

And that will prevent them all from going on two lines.

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