I've enjoyed successful upgrade bids on two Princess cruises this year, one earlier this month on the Royal with the Seawalk thing. I was bidding for an upgrade from an obstructed deluxe balcony to a mini suite. The only bidding options were for a minisuite or a Reserve (Club Class) mini. I didn't have the option to bid by specific category, and even if I had, they can always give you a cabin in a class higher than you bid for. I fully expected (if successful) to get a cabin under the Seawalk, which I think are MC category. I ended up in MC category but just forward of the forward elevators on Marina deck.
I usually choose to be aft of midship but actually quite enjoyed our location. We were lucky to be below the few Lido deck cabins so no noise from above. We moved to another mini on Marina for the second cruise of our B2B which was between forward and mid and was a lot noisier due to being under the Lido pool area.
I have plenty of photos if you need any.