How to create comic book word balloons in inDesign easily?

InDesign can do text frames like balloons pretty easily, but it will depend a bit on what export process you'll be using  — I note you tagged your post with EPUB, so if you're headed to EPUB or Kindle, there might be some limitations. But for print or PDF (which nearly always exactly mimics print), I'd use this —

• Create a text frame of about balloon size.
• In Object Styles, create a new style for this frame ("Text Balloon").
• Set the outside stroke to black and choose a width (1 point is probably a good starting point).
• Set the interior to white (you might want to add tinted options later, for thoughts/whispers/voice inflection).
• Click in the frame and add a few lines of text.
• Create a Paragraph Style for this text (begin with "No Prior Style," not any default styles).
• Set the text style up as you like with font, color, spacing, etc. (Again, you may end up creating a cascade of styles from this, but create your base, talky-talk one first.) Be sure to save/redefine the style so all settings are preserved.
• With the frame selected, hit Ctrl-B and set your inside offset values to center the text and keep it away from the outlines. You probably want the text centered (Paragraph Style) and vertically justified (Frame Options).
• In the top control bar, select corner style and set it to rounded. (Note other options for alternate speech, action descriptions, etc.) Set the corner size to give you the desired shape, probably a rounded rectangle.
• SAVE your Object Style (right click, Redefine).
• You can also start with an Ellipse frame for a fully rounded bubble. (Or a polygon one, again for alternate content.)

That should get you started. Create any new frame and apply the right Object Style as a start, then size and position it. Be sure to create and update an Object Style for each basic type.

As for a speech pointer or hook... you'll have to draw them yourself. A little curved V with white fill shouldn't be hard, and if you use the overprint method, where the V is distinct from the bubble, it's easy. To do single-outline bubbles, where the hook is integral to the outline... it can be done, but you'll have to master InDesign's pen and vector art tools. (Basically, put three anchor points on the frame and drag the center one out, then adjust position and, if desired, curves as you like.)

Yes, that's a lot of steps but most of them go into creating your models that can be reused over and over. No, there is no simpler, one-feature function that will do all this. 🙂