It's not filterable, but inside each color coded category it is alphabetized. First time this has been mentioned, to my recollection -- you know the backgrounds of the ship icon panels are color codes in addition to he icons themselves, right? Mentioning just in case. 
Yes I know they are colour coded.
2 things to mention about this.
What if your colour blind ? (Like one of my fellow players..) He has real problems distinguishing between them....
It is alphabetized, but it has it's issues.. See attachment. I'll use the Decloaker ship again. It puts them in with tachyon beam emitters.. Why?
So I would be looking at start of the list for "D", but it's right at the end as a "T".. And again as it puts them together it doesn't help me find the Decloaker as it highlights both ship types...
Also, you could just select all the ships on the planet and then use the textual selected ships window to filter down if you really want.
This maybe of help, if I knew what the "textual selected ships window" was.
Some additional comments.
My 2 friends and I, have been playing for 6+ months now, got all the expansions and
generally enjoy the game. However, we find that the UI and interaction with game to be quite unintuitive and awkward at times. See the example above. A simple search dialog box in both the galaxy map and system map would help me find any ship/constructor etc.. I could type decloaker or science or golem and it shows me their location on galaxy map, or highlights them on system map.. rather than the clunky searches available now.
Light of the Spire expansion has made the game so much easier, especially with the introduction of Spirecraft Martyr (Which seem incredibly over powered tbh). They remove the worry over almost any large incoming waves to just an annoyance, even a CPA. 3000 incoming ships.. put a martyr over wormhole.. bang goes the martyr.. mop up what's left of the wave (Normally not a lot).. Planet built up due to border aggression..? No problem.. Send in a martyr.. bang goes problem..
oh and another issue.. Warp gate attacks seem to be occuring from planets without a warp gate.. Is this a bug or can they happen from something other than a warp gate?