Der ganze Fisch mit Kopf und Augen, das sieht so wähh aus!
The "wähh" word over here is just an expression or has some meaning to it?
Also, is it a positive or negative expression?
The word wäh (not bäh or uäh, but wäh) is an interjection for expressing disgust. Like all interjections, it is colloquial to a certain degree. Nonetheless, it is a well attested word with a specific form and meaning. It is common in Switzerland. I do not know whether it is also used in other regions – it probably is.
The absence of wäh from common dictionaries or wordbooks suggests that it is not well known in Germany. The absence is rather scandalous since the word is well attested. However, the absence not unusual. Dictionaries and wordbooks tend to have a strong bias towards Germany – like the bias many English dictionaries used to have towards Britain a century ago. The Germany German equivalent would be igitt, which is found in dictionaries, though it also appears to be a regional word – it feels foreign in Switzerland.
The word wäh is even attested in the NZZ, one of the most respected German language newspapers, cf. articles like the following:
You could go on and find it in other Swiss newspapers that have less international prestige, but this already proves that it is a Swiss standard German word.
* One feature that distinguishes interjections from normal words is that the spelling is not necessarily fixed; see for instance pages 16–20 in this ruleset used in the transcription of voice recordings.
One thing stands out in the given example:
Der Fisch sieht so wähh aus!
Interjections are usually used independently. But in this instance, the interjection is used within a sentence like a normal adjective (e.g. eklig): modified by so and functioning as the complement of aussehen. I would associate this usage more with children or teenagers than adults.
As far as the phonetic form is concerned, there seems to be a certain amount of iconicity, although, as in all such cases, it is not quite certain what the precise connection between form and meaning is: The opening of the lips when producing bäh reminds me of spitting, whereas I connect uäh with the sound of vomiting.
Since Duden only has bäh, not uäh, I took a look at the Deutsches Referenzkorpus and found two nice examples:
Ich musste immer die Nieren putzen, waschen, schneiden, uäh. Das kann ich nicht ab. (die tageszeitung, 17.06.2017)
"Das ist voll gruselig", erzählte sie laut, "da stehen Hunderte Mumien herum. Ein paar sind schon richtig vergammelt - uäh! […]" (Berliner Zeitung, 03.06.2009)
As PMF explains in his answer, in the context of your question "wähh" expresses disgust. However, it is also used to ape weeping children. A nice example can be found here as the comment of the user fuschi:
Wähh wähh wähh! Uns gehts so schlecht. Wir müssen unfertige Spiele veröffentlichen, um DLC zu verticken. Wir müssen Project 10 $ und DayOneDLC einführen. Wir müssen den Multiplayer besteuern. Wähh wähh wähh! Wir leben an der Existenzgrenze!
Here are more examples found via Google: