It's not necessarily ridiculous, but it's certainly a little arrogant, and it's not good science to presuppose conclusions without evidence.
Which is exactly the problem with the ancient alien proposal - there's no hard evidence.
If you had a 5,000 year old building made with titanium, now we have something to talk about since, as far as we know, humans did not even know about titanium until the 18th century, and we could not produce it in a pure form until the early 20th. (FYI, this is one of the ways art historians can date paintings - they look for titanium oxide in the white paint).
Let's consider the problems with this idea:
1. Some alien culture discovered a method of interstellar travel that involved:
a way of reaching our solar system from another (a journey of at least 4 light years)
a method of shielding the crew from cosmic radiation (or a crew immune to its effects)
a method of protecting the ship from cosmic debris
a method of keeping the crew alive for the duration of the journey.
2. This alien culture had nothing better to do with our planet than pop down and build monuments to themselves. No mass extraction of resources, no colonization.
3. They managed to do all this without leaving behind more than anecdotal evidence about the whole thing. No signs of heavy construction equipment, no landing zones. (I mean if God descended from on high in your back yard, don't you think you'd kind of mark that spot and keep an eye on it in case He came back?).
As for the star child skull:
(from http://en.wikipedia...._skull#Analysis)