It requires some skill to get a presentation to look professional. And when you are a boot strapping startup, I think most investors will forgive you if you did not have time or money to get your investor slides look completely perfect. You decided to put your effort elsewhere, rather than spending it on PowerPoint looks.
So, the bare PowerPoint template with a tiny logo on the bottom right looks, well, bare, but you could still say it is professional, sort of. Worse is when you put in a lot of effort and the results do not look good:
- Clashing colours
- Too childish, or too cute for predominantly machine/male investors
- Tacky, cheesy stock images
- Super complex gradients and other template graphics that take over 50% of the slide surface
No format works for an early stage startup investor pitch with a good idea, it will not work in a sales presentation though.