是的，网友的句子是一个全部倒装句。这种倒装句是表语(主语补语)前置引起的全部倒装，多因尾焦(end-focus)，尾重(end-weight)，或二者兼而有之，而将主语置于句末，使之成为语义焦点，信息核心。所以，网友的句子把表语(主语补语)equally important前置于句首，句末的the mental aspect便成为了语义焦点，信息核心。

对于这种使主语成为信息核心的解释是有的。夸克等人的CGEL18.23有这样的解释:

… ln consequence，such fronting naturally carries with it the inversion that puts S in final position, and indeed it is to achieve end focus on the S that the fronting is generally undertaken:

CVS: Especially remarkable was her oval face. [1b]

CVS: Faint grew the sound of the bell. [2b]

AVS: ln a distant grave lies his beloved body. [3b]

此外，章振邦老师的《新编高级英语语法》46.4C特定情况下主语补语的前置也有说明(摘录部分内容)：

主语补语的前置通常并不引起倒装，但在某种情况下也可能引起倒装，这有时是因为主语较长或结构较复杂。例如：

More serious was the question of how the President would present the joint announcement.

有时为了上下文衔接紧密而把主语补语移至句首。例如：

What Uncle Conroy did was important, but more important was the way he did things.

表语(主语补语)前置引起的全部倒装这一语法内容，在Longman Grammar of Spoken and Written English 11.2.2.2 Fronted predicatives一节有比较系统的阐述。感兴趣地话，可以阅读。

最后补充一下Bas Aarts对end-focus和end-weight的解释：

1. end-focus: A term used in CGEL to refer to the placing of the most important information in a sentence or clause (the focus) at the end.

2. end-weight: A term used in CGEL to refer to the tendency of longer (i.e. heavier) units of information to come at the end of a sentence or clause. Since new information may need more detailed explanation than given information, end-weight often goes hand-in-hand with end-focus.

以上内容，仅供参考。

But equally important is the mental aspect.

=But the mental aspect is equally important.

备注:以上“属于表语强调”倒装.即为了强调表语而使用倒装结构。

Equally important is engaging in physical exercise

=Engaging in physical exercise is equally important.