It's possible the 53 could be of concern, but unfortunately it's really hard to say for sure without having any other data to analyze. Without other data points, we don't know if your kitty dropped more than that, or if 53 was the nadir.
Do you have a spreadsheet that we could look at? If you'll notice, everyone here has a spreadsheet link in the signature block. If you don't have one going yet, I really strongly encourage you to do so - it is the best way to track for sure how your kitty is responding to insulin. Additionally, we really can't give solid suggestions on what's going on without a spreadsheet.
Another bit of information that is important in helping us answer questions: What type of meter are you using? Human meters run a bit higher than pet meters, so knowing the type of meter is also important in analyzing the numbers. If you got that 53 on a human meter, it is a safe number, although just barely (we consider 50 to be the low end on a human meter); however, if the 53 reading came from a pet meter, then your kitty is definitely on too much insulin, as 68 is generally accepted to be the low end of safety on a pet meter.
PMPS = Evening (PM) Pre-Shot (PS). Likewise, AMPS = Morning (AM) Pre-Shot (PS). These are the fasting BG values taken before feeding and before shooting insulin. At the very least we strongly encourage testing before every shot to make sure it is safe to shoot, and at least one test during each "cycle", which is the 12 hours between shots. Many folks do work, but will grab a test when they leave the house to see what's what. In the evening, it's easier to test and at the very least getting a test before going to bed is a great idea to make sure kitty will be safe during the overnight hours.
Please keep asking questions. Everyone here is very generous with knowledge, support, and information.
And, finally, at the top of this forum are a number of stickies that have a TON of very useful and important information. Again, if you need help with the content of the stickies, just ask away!