Modding can definitely help you improve as a mapper, because it makes you look at maps more critically. You'll be able to see issues in your own maps more easily too. You can also learn new things from people's responses to your mods, so you should definitely read those.
As long as the mapper replies properly, posting misinformed mods won't waste your time, since you can learn from that mistake. It's a good idea to mod maps made by somewhat experienced people, because their replies will likely be more useful to you than new mappers' replies. It also reduces the risk of either you or the mapper getting weird misconceptions; that can easily happen when a new mapper mods another new mapper's map since they're less likely to know whether the other party's points make sense or not.
The catch here is that even though modding experienced people's maps is the most beneficial for you, it'll be a waste of time for them at first. Luckily, a lot of mappers are perfectly willing to "waste" some time helping a new mapper/modder improve. As long as you stay civil, I doubt most people will mind getting a few mods from a "random" new modder.
Bonus modding tip: always give reasoning for your mods. Not only will it make it easier for the mapper to understand your perspective, but it'll also help you make more informed decisions while mapping your own maps.