The first obvious issue is that the tenor and alto spend almost the whole first bar moving in parallel fourths. Using parallel fourths is one thing, but doing it in such an extreme way means the two voices basically blend to a single, overpowering one. You don't want the middle voices to team up in this way, they should generally be subordinate to the main voices. Especially with this being the first bar, it changes the whole perception/expectation/calibration of the listener, so even if the rest is fine then it probably won't come across.
Oh, and the melody itself joins the parallelism with that A-C in octaves. This should have been an obvious issue to you.
Then bar two starts with quaver-movement in all the accompaniment voices under the dotter crotchet in the main voice. To me, that dotted note has a pivotal role in the melody, and it's a point of rest, moreso than the minim on beat 3. With so much movement below it that definitely isn't the case anymore. Now, this could be fine – I'm not really sure if my interpretation of the melody is best. But you should have a good reason for doing that, and I don't see any here.
Aaron talked at length about bar 3. Honestly I find the mentioned doubled notes far less problematic than the other issues.
What does not work for my ears though is the subdominant in first inversion on beat 4, leading into... a sixth-only suspension of the dominant also in first inversion? IDK, is that even a thing? I'd say what definitely clashes is the sustained A in the melody with the G-A-B♭ in the tenor. And beat two seems to still try being a C chord, but for that of course the doubled B♭ puts the seventh way out of proportion, all the while there's still an A sounding in the soprano. No, bar 4 really isn't working for me, I guess you must have meant it completely different – but then you failed to make your intention clear in the score.
I wouldn't say any of those things are, per se, typical muddiness-causers. However, all together they are definitely prone to causing that effect, via at least two mechanisms:
- The voices don't have proper roles. That makes it confusing to the ear what should be tracked how.
- You have chords with either questionable inversion or unnecessary dissonance, that neither build up proper tension nor resolve to anywhere nor create any distinctive, interesting sounds.
In summary, this arrangement sounds like the voices occasionaly don't know what they're doing.