The weather in April in Mexico City, due to its elevation, is pretty temperate. The average is about 68 to 70 degrees fahrenheit. The evening may get slightly cooler. The minimum temperatures may reach the 50's but does not usually get any colder than this.
The April weather in Baja California North is similar to that of Southern California. You may get rain, though the rainy season is typically not as bad as in much of the USA's west coast. The temperatures are temperate with highs in the 70's and sometimes 80's and cooler nights, especially along the border towns like Tijuana. The climate is considered Mediterranean which means you have a wet cool season and a warm dry season. It also has many of the same variations, particularly inland, as California with steep drops in temperature that can get as low as 40 degrees in inland locations. The coast will rarely see such cool temperatures.
Baja California South has an average april temperature of mid 80's with 100 degrees being the record. The evenings, as with Baja California North, can get quite cool with an average being in the mid 50's and records being in the low 30's.
Central Mexico which includes Mexico City is subject to a variation of temperatures. In April you will see little or no rain as the rainy season begins in May. You will, however, experience a vast differentiation of temperatures depending on your elevation. Lower lying areas tend to be warm with temperatures reaching into the 90's. Night time temperatures do not fall below the 70's in lower elevations, but as with Mexico City, higher elevations remain quite cool and can even get cold toward evening and into the night.
Cancun boasts a very stable climate. The year-round average is in the mid to high 80's with little night variation and temperatures falling only into the 70's. It is an ideal time to go in April though it can get quite hot. The weather is a bit odd as you may find rain inland, while at the resort areas there may be sunny skies. The consistent climate makes Cancun a favorite for tourists.
Acapulco boast some of the best weather in Mexico with yearly average temperatures showing little to no variation. The days boast sunshine even though April is still officially rainy season. The highs are generally in the upper 80's to low 90's during the day and drop to the high 70's at night.
The busiest part of the year, or high season, is between December through April.
Cozumel, another popular tourist destination has two very rainy months, June and September. April does risk rain but generally is sunny and the temperatures average a high in the upper 80's to 90's and is humid. The nights are very temperate with averages being in the upper 70's.
Puerto Vallarta has two very rainy months, August and September. The rest of the year the sun is almost always shining. In April the weather is quite nice with highs in the 80's dropping to a comfortable 70's for the evenings and night. After May the weather can get much more oppressive this makes April a great month to visit as it is really the cut off point for optimal tourism.
Depending on the elevation and location the weather varies quite drastically. Generally the south of Mexico will be much warmer year round, aside from the higher elevations.
Central Mexico follows a similar pattern and the north part of Mexico also depends on where you go. The coastal areas, on both sides, are more temperate and lower lying which make them pleasant. The inland areas tend to be elevated or alternatively will be desert climates, that is to say these are cooler at night with the desert locations being very hot during the day and the higher elevated regions are significantly cooler at all times.
