Learn Dutch: Deze uniseks kleding is tamelijk goedkoop. - This unisex clothing is quite cheap.

Normally, an adjective before a de-word like kleding (especially after deze) gets an -e ending. However, uniseks is an invariable loanword. In Dutch, many modern loanwords (like plastic or vintage) and adjectives ending in certain vowels never get an -e added to them, no matter where they are placed in a sentence.

Would "goedkoop" get an "-e" if it were placed in front of "kleding"?

Yes. Unlike the loanword uniseks, goedkoop is a standard Dutch adjective. If you placed it before the noun, it would follow the normal rules and become deze goedkope uniseks kleding. In this sentence, it stays goedkoop because it comes after the verb (predicative use), where adjectives never get an -e.

How strong is the word "tamelijk" compared to other degree words like "heel"?

Tamelijk translates to "quite", "rather", or "fairly". It expresses a moderate degree, meaning it is less intense than heel or erg (very), but stronger than een beetje (a little). It is very similar in meaning and usage to the word vrij.

Is "kleding" a plural or singular noun here?