So I've been growing this plant for about two and a half weeks now and the undergrowth has just blown up... I'm wondering if it will be okay because it looks like it is blocked from the light
Looks good, no need to trim it now. Some plants will have more side branching than others and usually its a good sign that you have adequate lighting, at least for the time being. Its actually really great if you plan to lst , top, or scrog for a big bushy plant. If a lot of side branching doesnt fit in to your grow style then letting them grow just a bit makes a great place to take clones.
If it's not an auto I'd do some LST to get it to catch up and you can have a nice even canopy. What strain is it?