How salient is the graphic similarity of 戲, 謔, 虐 to readers?
In short, 虐 and 戲 are related to the character 虍/虎 in different ways, the former ideographically, the latter phonologically. According to Shuowen, 虐 in 謔 is but a phonogram.1 Shijing equating 善戲 with 謔 despite their derivational differences may thus well be a literary deliberation, which leads to Shuowen defining 謔 as 戲.1
虐 is a compound ideograph comprising 虎 (tiger) and 人 (man). Shuowen duly explains its origin as a tiger injuring a man with its claws. 2 虎 itself is a simple pictogram of a tiger.
戲, however, is a phono-semantogram. It comprises the phonogram 䖒 and the semantogram 戈, originally meaning 'secondary forces of an army'.3 䖒 is a type of ancient pottery that comprises the phonogram 虍 and the semantogram 豆.4 虍 is an abbreviated form of 虎.5
(Regarding how 戲 evolves to mean 'play; jest', there is debate. Perhaps it also means 'wrestle; fistfight' which then esoterically evolves to its current meaning. Perhaps there is loaning between 戲 and 嬉, but Jao Tsung-I argues 嬉 in the oracle bone script is a separate entity from 'play'. By Confucian times 戲 already possesses its current meaning.)
Consider a third character, 獻. Originally meaning 'a sacrifice dog', it comprises the phonogram 鬳 and semantogram 犬. 6 鬳 is a type of vessel that comprises the phonogram 虍 and the semantogram 鬲. 7
Oracle bone script of 虍: 
Oracle bone script of 虎: 
In fact, 虍 is a Kangxi radical. A cursory read at the characters sorted under 虍 convinces me that many of them are phono-semantograms that share the same phonogram 虍/虎. A notable exception is 虞, originally indicating a hypothetical tiger-esque beast, now meaning danger (a derived meaning). Obviously  虍/虎 now becomes the semantogram, and 吳 the phonogram. 8
Shuowen entries cited:
1. 謔：戲也。从言，虐聲。《詩》曰：善戲謔兮。
2. 虐：殘也。从虍，虎足反爪人也。
3. 戲：三軍之偏也。一曰：兵也。从戈，䖒聲。
4. 䖒：古陶器也。从豆，虍聲。
5. 虍：虎文也。象形。凡虍之屬皆从虍。
6. 獻：宗廟犬，名羹獻，犬肥者以獻之。从犬，鬳聲。
7. 鬳：鬲屬。从鬲，虍聲。
8. 虞：騶虞也。白虎，黑文，尾長於身，仁獸，食自死之肉。从虍，吳聲。


Edit

The etymological 'connection' between 戲, 謔, and 虐 does not matter much to the modern reader.

Above, I posited that the apparent connection between these three characters, if any, is at most coincidental, given 虍 acts as an ideogram in 虐 (tiger), but a phonogram in 謔 and 戲. The sole purpose for 虍 to exist in the latter case is to provide pronunciation, not meaning.

Many modern readers use the convenient (but often erroneous) strategy 有邊讀邊 (lit. to read one side of the character) when encountering a difficult character. 謔 I believe is no different; in fact the pronunciations of 謔 and 虐 are indistinguishable in Cantonese. However, that does not hinder the average reader from realising there exists a difference in meaning between them. Let us assume one does not know the character 銅 for 'bronze'; should they know how to read 同, they still will not jump to the conclusion that 銅 must share a semantic similarity with 同 for 'same'.

Hence my emphatic refutation of your statement; 謔 and 虐 should be seen as characters different in meaning. Of course there are a limited subset of characters where their phonogram is also their second semantogram (e.g. 坪), but I do not think 謔 is the case, nor does Shuowen.

Despite the marked difference in etymology between 謔 and 虐, I do concur with you that their immediate juxtaposition in the verse is intended to be appreciated, out of character similarity or pronunciational similarity. It just so happens that semantically the verse makes sense as well - a joke (戲謔) is good if it is not too far or 'cruel' (虐). This to me is, in a sense, a word play, or a literary tactic to attract the attention of the reader.

As a hypothesis, I do think 戲 and 謔 have evolved to be 連綿詞 despite their very different origins, so in that sense one may consider them 'connected'. No native speaker will, however, appreciate their graphical similarity (especially so if in simplified characters, sadly) - they each have their own meanings with which the phonogram 虍 does not associate; even if one does not realise 虍 serves such a function, it will still be conveniently ignored because it does not help in understanding the characters, let alone an ancient verse. That is to say, it has an almost fossil-like existence.

If by salient you mean overt, then yes, a reasonably educated reader will notice they share 虍, but that is a useless finding to most; if you mean noteworthy, then no, it is not, for reasons stated in points 3 and 6, and for the fact that this is a false proposition itself - there is no noteworthy etymological similarity between the three characters. Even if there were, the connection is very, very abstruse.