As long as the content is right in an investor presentation, you probably will get away for poor graphics design. Even if you did not manage to draw that banana perfectly, the audience will get the point. It even shows that you are a prudent CEO of an early-stage company not to waste money on expensive graphics designers*.
That changes with sales presentations. If you show up with poorly designed slides, you will lose credibility instantly, even if the content, visual concepts, and story flow are great. You come across as an immature, brand new startup (which you actually are), and that makes people scared to buy from you.
- Note of a professional presentation designer: getting the slides to look pretty from a graphics perspective is not the hard part, it is hard to get the content and concepts right.