Perhaps one of the most persistent misunderstandings of dyslexia is that one of its symptoms is perceiving letters as jumbled—especially as mirror-reversed, making letters like b/d and p/q especially challenging. However, the truth is that this is a very common thing for children to do when learning to read and write. With respect to dyslexia, its most common causes are unrelated to this mirror-reversal effect, or indeed to any difficulties with perceiving or producing single letters.
So, why do so many children write letters backwards? For adults for whom reading and writing is automatic and effortless, it is understandable that this seems like a funny, obvious mistake. But it turns out that the underyling issue is a difficulty with distinguishing between different perspectives of the same object. In a well-known anecdote illustrating this phenomenon (see Dehaene, 2009), it has been pointed out that “a tiger is equally threatening when seen or left or right profile” (Rollenhagen & Olson, 2000)—meaning, it’s often helpful to immediately recognize something regardless of which direction it is facing! Thus, the theory is that vision has evolved so that, by default, we perceive shapes like b and d or as the same object. This surely is beneficial in most circumstances, with letters being a notable exception (challenge: can you even think of other shapes or objects that have different names depending on whether they are facing left or right?).
Maybe one of the most interesting aspects of this is evidence suggesting that learning to read and write a language like English helps us learn to tell apart left and right. For example, illterate adults are slower to detect a visual difference between shapes like p versus q, as are fluent readers of languages that do not have such shapes that are reversals of each other (see Pegado et al., 2014; Kolinsky & Fernandes, 2014; Danziger & Pederson, 1998).
If you notice a child writing letters backwards, the good news is that this is common—in fact, most children will experience this a stage while becoming literate, and will cease to make such mistakes by around age 11. Of course, this leaves open the question of what are signs of dyslexia, and what might be its underlying cause or causes–you can read more about that here.