Logging time in an experimental aircraft that requires a licence issued under part 61 is legitimate time. Logging time in an ultralight that can legally be flown with no pilots licence is not legitimate.
I suppose you could. But why bother. An ultralight is very limited in usefulness. The only reason to have one is that you don't need a pilots license. It does not cost much more to build an experimental that is capable of carrying 2 people, goes faster, and is capable of flying longer distances.