Can Mälartåg trains become fully booked?

I'm planning to travel with a local commuter train and then transferring to a Mälartåg train to reach my final destination. To avoid wasting my ticket if the commuter train is delayed or cancelled, I plan to book the Mälartåg train last minute given that they have fixed prices anyway. Now, my concern is that I can tell this is a popular day to travel on and many trains by other operators are already fully booked. I have never seen a Mälartåg show up as fully booked, but that doesn't necessarily mean it can't happen.

Can Mälartåg trains end up fully booked and as such become impossible to buy tickets for on the Mälardalstrafik website?

No, they can not be sold out. As in at least most of Europe, local and regional trains do not offer seat reservations and there are lot of options to travel with these trains without having a ticket issued for a specific departure, e.g. travellers with a monthly pass or if an adult (even with a ticket for a specific departure) brings the two children, who can accompany an adult for free without having a ticket at all. In case of Mälartåg, they also operate lines where tickets from other operators, e.g. UL are valid.

Access to the trains is limited by a first come, first serve basis. It is at least not uncommon, that the trains are so full, that no seats are free and you have to stand. At least in theory, a specific departure may even be so crowded, that new passengers are refused to board. In this case, even if you have a Mälartåg ticket for a specific departure, you are allowed to use your ticket on the next train.