Yes, it is perfectly acceptable to strum with your thumb when playing an acoustic guitar. Using the thumb to strum is a valid technique and widely used by many guitarists, including experienced ones. While using the nails can produce a brighter tone, using the thumb can create a warmer and mellower sound. It's essential to find a strumming technique that feels comfortable and allows you to express yourself musically. Ultimately, what matters most is the sound you produce and the enjoyment you derive from playing the guitar. So, feel free to use your thumb if that's what you prefer.!