So what's would it be like to "travel in mayonnaise"? Flavors of Brazil isn't sure, but we have a feeling that it wouldn't be very easy, nor would one be able to see very much, thanks to that thick, oily emulsion of eggs and oil. Sludging though a landscape of mayonnaise would be like trying to make one's way through a wet, slick swamp, though granted that the smell and taste of mayonnaise is significantly more pleasant than that of swamp mud.
Although the expression is most commonly heard in Brazil as "travelling" in mayonnaise, there are other variants with the same meaning. You could accuse someone of swimming in mayonnaise (nadar na maionese) or even skating in it patinar na maionese. The action changes but the meaning doesn't.
We are at Flavors of Brazil are big fans of mayonnaise, as are most Brazilians, who love to put it on french fries, in sandwiches and hot hamburgers, and who's favorite salads employ a mayonnaise-based dressing. So, taking the name of the classic sauce to describe someone who's out of touch with reality seems somewhat unfair. If the expression requires a thick, opaque glutinous substance to make sense, we suggest that the expression be changed to viajar na arroz doce or "travelling in rice pudding." That way mayonnaise lovers needn't be unnecessarily offended.