Any ultra compact travel strollers that have good ventilation for tropical countries and budget friendly?
The Maclaren Atom has the most breathable seat and hood I have seen, to be honest, and the price, after some time, got to be quite nice. Also nice is to consider it pre-owned. The leclerc Hexagon has a breathable mesh on the seat's backrest - nothing huge, but at least something. Also, most ultracompacts, if having a peek-a-boo window or a hood extension, have a mesh part there so the air an flow in.
However, if you take ultra-compacts' open seat into consideration, and won't buy one that have very plastic/cheap polyester fabrics, you won't really need much extra ventilation in the seat. The best you can actually do is to get a breathable liner (e.g. Aeromoov) for the seat and dress your little one right for the weather. A more important feature for the tropics is, in my opinion, a canopy as large as possible.