If you're planning on staying in Lisbon for just one or two nights, then the Dom Carlos Park is... just fine. It's not nice, but it's not terrible either. If you're staying longer though (I stayed for five nights) then you'll probably regret it. Hotels in Lisbon are expensive and they've clearly gone with the market but the hotel is old, has basically nothing in terms of amenities and staff will charge you for absolutely everything. Do you need water? That's going to cost you. Did you finish your coffee pods after 3-4 days? They're going to charge you for me (3 pods given for a 5 day stay is absurd).
Did you want to iron your clothes? Forget that, they don't have irons although if you want to pay for it they have a laundry service (hope you don't need it on a weekend though).
The bed was stiff as a board and this caught up with me in a few days to be sure.
Breakfast was included and it was fine. But there are better at other hotels too.
In short, I have nothing good to say about the Dom Carlos Park. I will be back to Lisbon as I visit once a year but definitely not here.