Stupid (hope no offense by the abbreviation of your handle) - You are correct unless your flight lands early. IAH may not give you adequate time unless you clear immigration/customers/bag quickly as the connection across terminals is also a pain (long walkways, buses and trams).

However, IAH offers better services vs. LGA (think regional airport) - so if you do find 20 minutes - take it. Luckily, for IAH-LGA segment you don't need to get to date until 20 minutes prior to departure so don't rush unless necessary.